USPS Priority Mail Flat Rate boxes are an easy-to-use, cost-effective shipping option. You can easily ship like the experts; just know the USPS Flat Rate box sizes, prices, and how it works before you buy. How Does USPS Flat Rate Shipping Work? The USPS Priority Mail Flat Rate service will ship your package anywhere in the … Continue reading USPS Flat Rate Box Sizes, Prices, and How it Works