All Things Shipping | Jan 26, 2022

USPS First-Class Shipping vs. Shipping with USPS Priority Mail: When To Use Each

Sarah Rickerd

As an e-commerce seller, you know that every penny matters when it comes to staying profitable in today’s competitive markets. As such, you’re likely already familiar with two of the U.S. Postal Service’s most economical and accessible shipping options: First-Class Mail and Priority Mail.

But which one is the right choice for your business? Let’s take a closer look at the key differences between USPS First-Class shipping for e-commerce and e-commerce shipping with USPS Priority Mail.

What is USPS First-Class Mail?

If you’ve ever put a stamp on a letter and placed it in a mailbox, you’ve shipped using the USPS First-Class Mail service. Generally speaking, First-Class Mail is your most cost-effective option for mailing letters, postcards, and large envelopes, as well as small parcels.

That said, there’s more to USPS First-Class Mail than meets the eye. In fact, there are actually three separate First-Class services: First-Class Mail, First-Class Package Service (Retail), and First-Class Package Service (Commercial).

Shipping With USPS First-Class Mail

USPS First-Class Mail encompasses the following mail types and restrictions:

Mail Type Maximum Weight Minimum Size Maximum Size Thickness
Envelopes Up to 3.5 oz 3½” x 5” 6⅛” x 11½” 0.009” to ¼”
Postcards Up to 3.5 oz 3½” x 5” 4¼” x 6” 0.007” to 0.016”
Large envelopes or ‘flats’ Up to 13 oz 6⅛” x 11½” 12” x 15” ¼” to ¾”

All First-Class Mail pieces are paid on a per-ounce basis, based on the price of a First-Class Mail stamp. Items that fall outside of these ranges or that are considered to be ‘non-machinable’ may be subject to additional charges. E-commerce sellers shipping a large volume of First-Class Mail pieces may also be eligible for discounted commercial rates.

But, for Shippo users, discounted USPS rates can be accessed immediately and for free after signing up.

First-Class Mail services have an estimated delivery time of 1-5 business days—day-definite delivery is not available with this service. Tracking is not available for First-Class Mail shipments, though delivery confirmation services such as Certified Mail may be available.

First-Class Package Services: Retail vs. Commercial

Small parcels can also be shipped via USPS First-Class Package Service (FCPS), on either a Retail or a Commercial basis. 

(Note: Shippo users only get access to FCPS on a commercial basis which allows them to ship slightly heavier parcels without incurring an additional fee or changing the delivery service.)

Retail Commercial
Minimum Size 3½” x 5” (0.007” in thickness) 3½” x 5” (0.007” in thickness)
Maximum Size No more than 108” in length and girth combined No more than 108” in length and girth combined and no more than 18” x 22” (15” in thickness)
Maximum Weight 13 oz 15.99 oz

Like First-Class Mail, First-Class Package Services are typically delivered within 1-5 business days and are priced on a per-ounce basis. Unlike First-Class Mail, however, First-Class Package Services (both Retail and Commercial) take pricing zones into consideration, so your shipping costs may vary based on the distance between your origin zip code and your destination.

Visit this page for more on the differences between First-Class Retail and Commercial Package Services, as well as for restrictions on what can and can’t be shipped via First-Class services.

What is USPS Priority Mail?

For this article, we’ll look at two popular USPS Priority Mail services: Priority Mail and Priority Mail Flat-Rate.

(Note: Priority Mail Cubic, Priority Mail Express, and Priority Mail Regional Rates are also available for Shippo users. Priority Mail Cubic features our highest discounts with up to 89% off certain shipments).

Priority Mail pricing is determined by the package’s weight, size, and destination zone. Priority Mail Flat-Rate, on the other hand, allows e-commerce sellers to use USPS-provided packaging and pay a single, set fee for any item(s), up to the service’s weight limit.

Like First-Class Mail, USPS Priority Mail allows e-commerce sellers to send both envelopes and parcels. In addition, both First-Class and Priority Mail can be used to ship items both domestically and internationally (delivery times increase for international shipments).

However, there are a few differences you’ll want to be aware of:

  • Priority Mail is typically faster than First-Class, shipping in an estimated 1-3 business days, rather than 1-5 days.
  • Priority Mail can be used to ship much larger packages (up to 70 lbs for some services).
  • USPS Tracking and a small amount of insurance is typically included with your shipment.
  • It is typically more expensive to ship items by Priority Mail than by First-Class services when shipments are under a pound.

Although there are several distinct services within the USPS Priority Mail Flat-Rate program, some of the most common services e-commerce sellers use include:

Inside Dimensions Outside Dimensions
Small Flat-Rate Box 8 5/8″ x 5 3/8″ x 1 5/8″ 8 11/16″ x 5 7/16″ x 1 3/4″
Medium Flat-Rate Box (top-loading) 11″ x 8 1/2″ x 5 1/2″ 11 1/4″ x 8 3/4″ x 6″
Medium Flat-Rate Box (side-loading) 13 5/8″ x 11 7/8″ x 3 3/8″ 14″ x 12″ x 3 1/2″
Large Flat-Rate Box 12″ x 12″ x 5 1/2″ 12 1/4″ x 12 1/4″ x 6″

Priority Mail Express services are also available for items that need to be delivered either next-day or within two business days.

How to Choose Between Shipping With USPS First-Class Service and USPS Priority Mail

Given the variability between these services, there are several things you’ll want to consider when selecting the right one for your e-commerce shipments:

  • The size and weight of your packages
  • The distance between you and your buyers
  • How quickly you need your items to be delivered

Let’s take a look at how these differences play out in the real world with an example. Imagine that you sell custom-printed t-shirts that weigh about six ounces each when packaged, and that you need to ship a single shirt from the New York zip code 10001 to the Chicago zip code 60610.

As of this writing, here’s how the different services described above break down in this example:

Retail Cost Estimated Delivery Time Free Packaging Provided?
First-Class Package Service (Retail) $5.25 4 days No
Priority Mail $9.85 3 days No
Priority Mail Small Flat-Rate Box $9.45 3 days Yes

In this example, if cost-savings is your biggest priority—even if it means sacrificing speed—First-Class Package Service may make the most sense. Interestingly, if speed is a bigger priority, it’s still important to crunch the numbers. It’s also worth noting that the USPS Priority Mail Flat-Rate box was cheaper than Priority Mail, even though you have the added benefit of reducing your packaging expenses with the USPS-provided box.

That said, these calculations change if you need to ship longer distances or if you’ll be mailing more than one t-shirt at a time. Always double-check to make sure the shipping service you choose represents the best value for you and your customer’s particular needs.

Having a shipping solution such as Shippo allows you to compare USPS rates for every shipment so that you’re always choosing the right shipping service.

Access Discounted USPS Shipping Services with Shippo

It’s important to note that the rates given above reflect what you’ll be charged if you purchase postage directly from the USPS. You can knock those shipping costs down even further by working with Shippo.

Because of our pre-negotiated account with the U.S. Postal Service, e-commerce businesses can join and immediately get access to discounted shipping rates for USPS. Discounts can be found when printing out shipping labels for USPS Priority Mail®, USPS First-Class Mail®, and many other popular USPS shipping services.

In some cases, e-commerce merchants will be able to create USPS shipping labels for up to 89% off of retail rates.

Sarah Rickerd is a Michigan-based freelance writer covering business, marketing, and technology topics. She is also the owner of Content Conquered, a B2B-focused content marketing consultancy.

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