Customs don't care if you've paid for the goods or not. Their charges are simply based on the value of the goods, and you have to declare the value that they will calculate duty and VAT on.

Any goods imported from outside the EC are deemed to have a value for Customs purposes, whether bought, gifted or stolen :-) and they will want their pound of flesh - oops, sorry, I meant duty and VAT.

Currently I believe that it's 4% duty and 17.5% VAT (on the value declared plus the duty plus any shipping costs).