Not to mention, I'm sure the losses incurred by not supporting traditional payment are far, far more than 3%. You have kind-hearted users who are opening their wallets, ready to donate money, but then you're telling them that you can't accept their form of money. In most cases these (lazy) users will, at this point, give up and leave.

credit card fees are not 3%, they are 3% + 30 to 40 cents, which effectively make credit cards ineffective for transmission of anything < 50c.