Sí, parece que es medio común ese problema :S.
"I have the exact same problem as you, there's also a thread on the GMG forums where a ton of people are having the same problem. The only solution that seems to work, is to have GMG Support answer your ticket and then reset your account. I've tried both my CC and paypal accounts, submitted multiple tickets after the first ones apparently no longer exist. Tried buying the game on Chrome/Firefox/IE, cleared cache/cookies and still no go. I also contacted radaeron here on reddit about it, but since he doesn't have anything to do with billing, he can't really help.
Its killing me to miss this deal since I also have $12 of credit on GMG that I now can't apparently use. Been saving this credit forever too. Only hope is now after whats prolly been a crazy weekend for GMG, tech support gets back to me in time and fixes my account so I can get Borderlands for $28 after coupon/credit."
Fuente: http://www.reddit.com/r/GameDeals/comments/x9vhm/is_anyone_else_unable_to_buy_games_from_green_man/