if the shop is so small and cannot afford the cc changes for amount below X $ then why sign up to be cc merchant then?
on the other hand if the charge amount is too much to absorb, then add it to the price of your product/service.

putting the "visa/mastercard accepted" sign only to turn around and say we accept for amount above X is totally rubbish. its akin to bait and switch.
only thing worst than this are those PC shops.. yes we accept creditcard but we will charge you extra X% (was it 10%?) total daylight robbery