USPS Flat Rate vs. WhatNot Flat Rate — What You Need to Know At first glance, USPS flat rate shipping and WhatNot’s version of flat rate shipping seem like the same thing but they don’t actually play by the same rules. USPS Flat Rate The benefit of USPS flat rate boxes is simple: if it fits, it ships. As long as your box is under 70 pounds, you can ship it for the flat rate price, no matter the actual weight. That’s the true advantage of using these boxes. WhatNot Flat Rate WhatNot adds its own