All Things Shipping

Jul 15, 2026

Poly mailer sizes, mil thickness, and carrier rules

A poly mailer is a lightweight, water-resistant polyethylene bag that USPS, UPS, and FedEx all ship in place of a box for soft goods like apparel, cutting postage cost and storage space compared to corrugated cardboard. This guide covers sizing, the mil thickness that actually determines durability, real shipping rates, carrier rules, and when a box still beats a bag.

What is a poly mailer made of?
Poly mailer sizes: which one do you need?
What mil thickness should you use?
Poly mailer varieties
How much do poly mailers cost to ship?
Carrier rules and restrictions for poly mailers
Sealing a poly mailer the right way
Poly mailers vs. bubble mailers vs. boxes
Are poly mailers recyclable?
Can poly mailers be customized?
What's new in 2026: USPS Cubic pricing for poly mailers
FAQ: poly mailers

What is a poly mailer made of?

Poly mailers are made of polyethylene, a light, synthetic resin that's the most widely used plastic in the world — the same material behind shopping bags, food wrap, and detergent bottles. Most standard mailers use low-density polyethylene (LDPE) film, which gives them their stretch, tear resistance, and water resistance without adding weight to the package.

Poly mailer sizes: which one do you need?

A poly mailer doesn't carry its own official weight limit. The limit that applies is whichever shipping service prints the label, not the bag itself. USPS Ground Advantage, the most common service for mailers, caps out at 70 lb and 130" in combined length and girth — far more than any poly mailer realistically needs to carry. Size is the bigger constraint in practice: pick a mailer that fits the product snugly, since an overfilled or underfilled bag causes more shipping problems than a slightly heavy one.

Here's what trips people up when measuring: on a mailer labeled "10×13," the first number is the width (the seam-to-seam measurement across the opening) and the second is the length (top to bottom). Measure your product's widest point against the width, not the length, when sizing up.

Here's a general guide by size:

Size	Could ship soft goods like:	Could ship boxed goods like:
Small Poly Mailer 7.5×10.5"	2–3 rolled up shirts 1–2 winter hats or 1 baseball hat 3–4 pairs of socks A folded scarf or towel	Accessories like earrings, necklaces, bracelets, or pins Small non-fragile ornaments or decorations Collectible coins or trinkets
Medium Poly Mailer 10×13"	A pair of pants or shorts Multiple shirts Multiple winter hats or 2–3 baseball hats (stacked) Multiple scarves	Books Small or medium-sized artwork Non-perishable boxed foods like candy
Large Poly Mailer 14.5×19"	1–2 sweatshirts or jackets 5–8 shirts 2–3 pairs of pants or shorts	A pair of shoes Well-packed artwork Bulky apparel items like large, fluffy sweaters

What mil thickness should you use?

Carriers don't set a minimum mil thickness. Whether a poly mailer survives transit comes down to the film thickness you pick. One mil equals one-thousandth of an inch, and puncture resistance increases meaningfully with it — a sharp corner is far more likely to punch through 2 mil film than 2.5 mil. Match the mil to what's inside, not just the size:

Mil thickness	Best for	Why
1.7–2.0 mil	Folded apparel, soft goods with no sharp edges	Lightest option; fine when nothing inside can poke through
2.5 mil	General apparel, mixed soft goods (the common default)	Noticeably better puncture resistance for a small cost increase
3.0–3.5 mil	Shoes, boxed cosmetics, items with hangers or rigid inner packaging	Rigid edges inside a thin mailer are the top cause of in-transit punctures
4.0+ mil	Heavier bundles, hardware, multi-item shipments over 2 lb	Added durability for weight and repeated handling

Several packaging retailers advertise a USPS minimum mil-thickness requirement by weight (for example, "2 mil up to 5 lb, 4 mil over that"). That figure traces back to an older USPS mailability standard for plastic bags and wraps, not to Notice 123 — the document that actually sets Ground Advantage pricing and size/weight eligibility. Notice 123 doesn't tie your rate or your carrier eligibility to material thickness. Thicker film protects your shipment from punctures; it's a durability choice, not a requirement for using USPS.

Poly mailer varieties

There are several types of poly mailers to choose from, depending on what you're shipping:

Type	Best for	Note
Layflat	General apparel and soft goods — the flat plastic bag you've probably received from most online orders	The industry standard; seals with a self-adhesive strip
Clear View	Print materials like catalogs, brochures, and magazines	Transparent on one side with an opaque backside for postage and labels
Bubble-Lined	Fragile goods that don't need a full box	Built-in cushioning at a lower cost than a padded box; usually self-sealable
Expansion	Bulkier items like jackets, sweatshirts, books, or binders	Expandable seam along the side makes it easier to fill without stretching thin
Returnable	Any product where returns are common	Two self-seal adhesive closures so a customer can reseal and send the same bag back
Recycled	Sellers prioritizing lower footprint	Made from post-industrial and post-consumer material; disposal path is the same as standard mailers

Returnable mailers pair well with printing a prepaid return label upfront — see Shippo's guide to sending a prepaid USPS return label — so customers can send items back without a trip to buy their own packaging.

How much do poly mailers cost to ship?

Poly mailers keep dimensional (DIM) weight low. That's most of why they cost less to ship than a box carrying the same item: light, thin packages qualify for cheaper service tiers that a rigid box often can't reach. Here's what that looks like using two of the sizes from the table above:

Mailer size / weight	Cheapest service	Rate	Transit
Small (7.5×10.5", 4 oz)	USPS Ground Advantage	$5.97	5 days
Large (14.5×19", 1 lb)	UPS Ground Saver	$8.76	5 days
Large (14.5×19", 1 lb)	USPS Ground Advantage	$10.01	5 days

Rates are Shippo commercial rates pulled July 15, 2026, for a shipment from San Francisco, CA (94117) to New York, NY (10001). Actual rates vary by your account's negotiated pricing, origin and destination, and carrier fuel surcharges. Use Shippo's rate calculator to check current pricing for your own shipment.

The cheapest option shifts by size and weight. That holds true for comparing USPS, UPS, and FedEx rates generally, so it's worth checking live rates across carriers for each shipment rather than defaulting to one.

A poly mailer doesn't hold a rigid shape, but carriers still measure and price dimensional weight the same way they would for a box — which surprises a lot of sellers the first time they see the bill. An overstuffed mailer with excess trapped air can quote higher than the snugly packed version of the same product. Pack it tight to keep the DIM weight, and the rate, as low as the material allows.

Carrier rules and restrictions for poly mailers

Poly mailers ship fine with all three major U.S. carriers, but each has rules that affect eligibility or cost once a mailer gets larger or heavier than the smallest sizes.

USPS doesn't require a minimum thickness or rigidity for parcels shipped as Ground Advantage packages, so a standard poly mailer qualifies without special handling. Nonstandard-length fees do apply once a package runs long, whether it's a mailer or a box: $4.50 for 22 to 30 inches, $10.00 for over 30 inches, and $21.00 if it exceeds 2 cubic feet, per USPS Notice 123.

UPS and FedEx handle oversized soft-sided packaging differently than USPS. Both apply an Additional Handling surcharge once a poly bag or bubble mailer exceeds certain dimensions — commonly cited as around 18 inches on the longest side and roughly 5 to 6 inches in height, with a typical surcharge in the $25 to $35 range depending on zone. Both the exact trigger dimensions and the dollar amount vary by account and change with each carrier's annual rate update, so confirm current accessorial rates directly with UPS, FedEx, or your account rep before budgeting around a specific figure.

A standard large poly mailer (14.5×19") typically stays well under these thresholds, but an overstuffed one won't. Standard poly mailer sizes rarely trip these surcharges — stuffing a mailer past its intended capacity to avoid a box can end up costing more than the box would have.

Sealing a poly mailer the right way

The built-in adhesive strip on most poly mailers is a hot-melt adhesive, and it can lose tack in cold weather or after sitting in a warehouse for a while — the classic "why won't this stay sealed" complaint. Static cling from the polyethylene film itself can also make tape and labels harder to seat cleanly, especially in dry conditions.

Try these instead:

For cold-season or high-value shipments, reinforce the self-seal strip with a strip of packing tape across the flap and down both sides of the bag
Wipe the sealing area if there's visible static or dust before pressing the strip closed — it's a fast fix and it works
If you're printing custom mailers, ask your vendor about acrylic adhesive instead of hot-melt; it holds tack across a wider temperature range

Poly mailers vs. bubble mailers vs. boxes: which should you use?

Poly mailers win on cost and storage for the right products. A stack of poly mailers takes up far less space than the same number of collapsed cardboard boxes, and a mailer skips the assembly a corrugated box requires. Bubble mailers sit in between: more protection than a standard poly mailer, less bulk than a box.

Factor	Poly mailer	Bubble mailer	Box
Shipping cost	Lowest — thin, light packaging keeps DIM weight down	Slightly higher than a standard poly mailer	Highest for the same contents once DIM weight kicks in
Storage space	Flat and stackable	Flat but bulkier than a standard mailer	Takes up the most room, even collapsed
Fragile items	Not protective on its own	Light cushioning — good for small, delicate items	Best protection with void fill
Liquids or leak-prone items	Water-resistant, not leak-proof — not a substitute for sealed primary packaging	Same limitation as poly mailers	Best when paired with a sealed inner container
Weather resistance	Water and tear resistant by default	Water and tear resistant by default	Needs a liner or coating for the same protection

A box is still the better call in a few situations. Breakable contents need real void fill, not just a bubble-lined mailer — check whether shipping insurance would cover those breakage claims if they're cutting into margin. Liquids and cosmetics in glass or unsealed containers shouldn't go in any mailer; poly and bubble mailers are water-resistant, not leak-proof, so a spill inside the bag is a spill in transit. And a product that doesn't sit flat in any standard mailer size creates gaps or flat spots that carrier scanning equipment can misread, leading to sorting errors, delays, or a Return to Sender; pay the extra cost for a padded box like the ones used for shipping books in that case.

Are poly mailers recyclable?

Most poly mailers are made from LDPE or HDPE plastic film, and most curbside recycling programs don't accept them because the film can jam sorting equipment at material recovery facilities. Poly mailers instead need to go through a store drop-off program, the same bins many grocery and retail stores use for plastic shopping bags, per How2Recycle's guidance. To qualify, the mailer needs to be clean, dry, and free of paper shipping labels before it goes in the bin.

"Compostable" poly mailers are a real product, but the claim needs a caveat: they require industrial composting facilities, not a backyard compost pile or a landfill, to break down as advertised. Access to those facilities is limited, and a compostable mailer tossed in a plastic-film recycling bin contaminates that batch instead of helping it. If sustainability packaging claims matter to your buyers, verify your mailer's actual end-of-life path before marketing it as eco-friendly.

Sellers shipping into California should also know that the state's Extended Producer Responsibility law (SB 54) now covers e-commerce shipping materials, including mailers and envelopes, under permanent regulations that took effect May 1, 2026. It doesn't change how you ship day to day, but it does put new recycled-content and reporting obligations on the packaging producers you buy from — talk to your supplier directly if you're sourcing mailers at volume.

Can poly mailers be customized?

A 2018 Paper and Packaging Board survey found 72% of consumers say packaging design influences their purchasing decisions, giving merchants a real opening to make an impression with something as simple as a mailer.

Poly mailers are easy to customize and can help reinforce brand loyalty. Here's how:

Choose a size that fits your shipment
Change the background color to match your brand
Add a logo or a catchy design
Feature compelling or witty text that engages customers

Minimum order quantities for custom-printed mailers vary widely by vendor — some print runs start as low as a few dozen units, others require several hundred. Get a quote before committing to a design if order volume is a constraint.

What's new in 2026: USPS Cubic pricing for poly mailers

Effective July 12, 2026, USPS restructured Ground Advantage Cubic pricing for softpack shipments, which includes poly mailers and padded envelopes. The tier count dropped from ten to five, and the measurement method now aligns with Priority Mail Cubic, pricing off combined length plus width instead of the old tier structure. High-volume shippers who qualify for Cubic pricing should run a fresh rate check even if nothing else about their packaging changed. See Shippo's guide to USPS Cubic pricing for how the tiers work and whether your shipment volume qualifies.

FAQ: poly mailers

What size poly mailer do I need? Match the mailer to the product for a snug fit; that ships more reliably than an oversized bag with excess air inside. Use the size table above as a starting point, and size up only if the product doesn't fit without folding awkwardly.

Can I ship a poly mailer with UPS or FedEx? Yes. Both carriers accept poly mailers, though each charges an Additional Handling surcharge once a soft-sided package exceeds the thresholds in the carrier rules section above.

Does USPS require a specific mil thickness for poly mailers? Not for Ground Advantage. Notice 123, the document that sets Ground Advantage pricing and eligibility, bases both on size and weight, not material thickness — the "2 mil up to 5 lb, 4 mil over that" figure some retailers cite comes from an older mailability standard, not a current shipping requirement. Choose your mil based on what's inside (see the thickness table above), not a carrier rule.

Is it safe to ship liquids or cosmetics in a poly mailer? Not on their own. Poly mailers are water-resistant, not leak-proof. Liquids and unsealed containers need a sealed primary container at minimum; a box is the safer call for anything that could leak.

Why won't tape stick to my poly mailer, and how do I fix it? The built-in adhesive strip is usually a hot-melt adhesive that loses tack in cold weather, and static from the film itself can make tape harder to seat. Reinforce the seal with packing tape across the flap and both sides of the bag for cold-season or high-value shipments.

Shippo can help

Compare live rates across 40+ carriers, including USPS, UPS, and FedEx, for your next poly mailer shipment with Shippo.

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