No, the shop does not get paid in installments, they get the full payment after a few days, less the banks' commission.
Yes, it is forbidden as per contract. But smaller shops do not abide by this. If they are earning very little, and the customer wants installment, then they can discuss it with the customer. The shop can of course refuse not to sell on installment, it's just a service offered, not mandatory. Of course, they can also just add the commission to their mark-up, but some shops may want to appear to be competitive with other shops, especially those without such a service, so they don't. I guess it's a cutthroat industry, so many photography shops around. Of course, it exists in non-photography related stores. It is so common, it's already part and parcel of shopping :dunno: Of course, if you buy at the bigger chain stores, the commission is already factored in, and they sell in large volume anyway that it doesn't matter. They can also buy in large volume, so they get better prices from manufacturers, usually.