us postal service if it fits it ships rates refer to USPS’s flat-rate shipping model, where the price is determined by the box size, not the weight (up to the maximum allowed). This slogan popularized Priority Mail Flat Rate services, offering predictable pricing and free boxes. The convenience of us postal service if it fits it ships rates benefits small businesses, eBay sellers, and customers sending care packages. It simplifies the decision-making process for shipping and reduces the need for weighing or complex rate calculations. However, it’s essential to compare flat rate vs. regular pricing to ensure cost efficiency.