USPS Flat Rate vs USPS Priority Mail: Flat Rate Boxes, Prices, and Services. USPS Flat Rate is a commonly used shipping service. A major reason this shipping method is used so frequently is because it's a very easy parcel size for the USPS to fit into their trucks. It's also a good way for the USPS to make money.

Do you have to use a Flat Rate envelope?

The only time you have to use a Priority Mail-branded box is if you're shipping a Flat Rate box or envelope. ... For example, you have to use a Priority Mail Small Flat Rate Box® if you purchase Priority Mail Small Flat Rate Box® postage.

Is it cheaper to use your own envelope for priority mail?

While Medium and Large Flat Rate boxes may be free, using your own packaging is often CHEAPER than shipping Priority Mail Flat rate, thanks to a “secret” USPS mail class called Priority Mail Cubic.