Why has FedEx sent me a bill?

When FedEx imports your products for you, you may owe import duties and other taxes on that delivery depending on where you live. Unlike the post office, FedEx does not force you to pay those fees before they'll give you your package. Instead, they pay them for you, and then bill you later. This bill will need to be paid to FedEx, just as you'd pay the post office or their delivery agent for import duties or other taxes on delivery.

You can read more about import duties and taxes here.