You should have refused to pay it. Charging a fee to a customer for using a credit card is a violation of Pay Pal's, Ebay's, & ALL of the bankcard companies user agreements. It is also against the law in some states.

Bankcard merchant fees should be considered a cost of doing business by the seller. If a seller isn't willing to pay the fees, they either shouldn't be in business or should quit accepting credit cards. IMHO
Bill R-4581