Amazon is the big online gift cert place, I think. When I went to go visit a U.S. panty girl (Mzleathercorset on Ebanned smt ) I had a client pay us through a purchase at giftcertificates.com (she and I had a lovely dinner out together!)
I’m still a cash-in-the-mail kind of seller. I can spend it anywhere, and if your buyer is a trusted repeat you can extend “credit” to speed a panty purchase along – it’s not as cumbersome as you might expect. 🙂