If you don’t, USPS will charge you later for the proper service at the Retail rate. Pro Tip: Always make sure you pay for the specific service marked on your USPS branded box. We’ve listed out the different types of Flat Rate boxes and envelopes below: Each type of packaging is priced differently, and you can ship up to 70 pounds in each of them. The main thing you need to know about Priority Mail Flat Rate is that it comes with several different types of packaging that you can use. The Different Kinds of Flat Rate Packaging
Priority Mail Flat Rate is a popular service offered by USPS.