As a merchant, I never understood the "sales under $10" rule. The percentage charged by the card
company is the same no matter what the total. Businesses that decide to take credit cards should already expect this percentage. There is about a 20-cent charge per credit transaction. I was never willing to inconvenience a customer for 20 cents!
For debits the per-transaction fee is a little higher but still under $1.00, and there is no percentage charged.
The terminals used to be quite expensive but still, you're paying monthly whether you use them or not. They've also gotten much cheaper in recent years.
Actually, the "under $10" rule is in violation of most of these contracts so you'll notice businesses soft sell it, as in "please respect our minimum $10 purchase, etc." It's only a request.
Now, when it comes to refunds, the business is charged the percentage coming and going, so if you return something it costs the merchant double the processing fee, about 5%, plus two transaction fees. Small boutiques can't afford returns the way department stores can. Jacques won't give refunds at his store on credit card purchases--just store credit. People get quite irate even though it is posted at the register.