Much like there is a difference between retail and wholesale pricing, so too are there differences in how rates are calculated for shipping.
Most of the time these are based on how much you ship with a certain carrier. Or what technology you use. Or what type of packaging you use.
Rates with the United States Postal Service are perhaps the most cut-and-dry while also being the most complicated. There are three basic tiers of rates with the shipping carrier (in order from most to least expensive): Retail, Commercial Base Pricing (CBP), and Commercial Plus Pricing (CPP).
Much like with USPS, both UPS & FedEx have their own tiers of rates, though they are a bit less clear-cut. Generally you have two tiers: retail rates and negotiated/contract rates.