One my friend doesn't have a Credit Card but want to buy good old games. Pay per call can be an interesting solution for payment of such a small-priced games, imho.
What do you think about it?

It's difficult to implement and confirm that the one who pays is the one who downloads the game. Why don't you just buy it for him and he gives you the money?
Or another option: he creates a paypal account with no credit card associated, you put money on his account, he pays you back, he buys via paypal. This way he is the owner of the game and not you.