McMurdo has them, but the general store accepts cards. I don't think the bars accept cards and tipping for volunteer services (e.g. barristas) is quite common. Pole does not (it's cash only). And then there's Palmer which is card only. Most contractors can "widthdraw" money out of their paychecks to get a periodic stipend while they're down there, otherwise you have to take out a big wad in McM to last the summer/winter (e.g. scientists usually can't because we're paid by universities, not USAP).