Debit card payments (in Germany at least) are as free as it gets for the merchant, and they get their money immediately (not several days later like with credit cards).
(I still prefer cash.)
That being said, speaking from personal experience owning and running retail stores, credit cards are a pain to accept because of the transaction fee vs. microtransaction issue when people decide to pull their credit cards out for all of a $2 purchase, but more customers spend more money usually because they didn't have any cash on them. What can you do....heh.
Do you have any experience with debit cards vs credit cards?
> ...update from the real world.
All things considered, I don't think there's anything fundamentally different about physical purchases vs. online purchases made with a credit card. The only big one is that micropayment systems and fees aren't really in place for brick and mortar retailers that might struggle just to accept cards in the first place, whereas it's becoming more and more common online because people are buying 99 cent songs and apps all over the place. It needs to happen for the former because people make 99 cent purchases everyday anyway (usually combined with something else, or else they're paying with cash...hopefully), but that's a problem that hasn't even been sufficiently solved in an all-digital world that loves the idea but can't easily do it because of all the damn fees.