Sellers with the Professional plan set their own shipping rates (except for books, music, video, and DVD products). When a customer buys your product, you receive a credit from Amazon that matches the shipping rate. Keep in mind that the referral fee is calculated from the total sale price, including the item price, shipping cost, and any gift-wrapping charges.
Sellers with the Individual plan use Amazon's set shipping rates for all products. Individual Sellers have to ship orders at the set rates even if the shipping credit is less than the total shipping costs, so it's important to price in a way that makes sure you'll earn a profit.