As of June 1, 2013 if you happen to have a United Airlines Milage Plus card, you will not be charged a forgein fee charge on any purchase any longer. Since United is basically the only US carrier that flies into Huatulco it may be beneficial to obtain one, in addition to other perks it has,as not being charged for a second bag for you and anyone else booked on the flight ,if you purchase those tix with the card.