The way I understand it, Fedex is authorized to collect the various duties and such. Now, technically, these duties should always be collected, it's just that other methods often slip through. Fedex makes sure you get charged the duties, then charges you another fee for the effort required to act as the customs agent.

You can avoid these duties by ordering from US suppliers. Even if the product actually comes from out of country, these suppliers already include the cost in their price. If you order from out of the country, be prepared to pay the duty.