I guess the change in the law is significant. However, I thought that shop owners would assume that any large item would be paid for with a credit card and raise the price accordingly. I don't have any evidence for this, but, is it likely that the only effect of this ruling will be on items that are usually paid for with cash, ie those under $25 (or whatever reasonable number of dollars)

I never knew that stores cannot charge more. I would assume more stores will already set the price with credit card charges in mind. Some restaurants give you 5% discount for paying in cash.

It's a loophole of sorts. They cannot legally post a sign saying "Credit card transactions will have a 5% fee added" but it's perfectly fine to say "Cash purchases receive 5% discount."