I don't think the law comes into play in Canada for this issue. It's just a matter of merchant agreements. For example, it is against the VISA merchant agreement to charge a service fee for use of their product (the credit card). There is nothing to prevent a store from offering a cash discount however. And this is the loophole every small computer store has used for the past 20 years. You'll see the fine print on advertisements "prices are cash-discounted 3%"

I've never seen anyone with a policy of surcharging debit-card (ATM card) purchases. I've only used a debit card at a convenience store a couple of times though.

Bruno