Brake arms are not precision equipment. Anyone...accurately measure between the arms when they're extended. If they actually measure what they claim to be, you've got one in a million. In any case, it is no problem to shave a millimeter or so of plastic off the brake arms. The arms can be bent, but they're made of steel that is designed to not bend easily, so bending to size can be a chore.
Dishonest shop?...how can that be? Is it like the shops that charge the full binding installation price to slide system bindings on to the rails already on new skis? Do pay for setting the release adjustment if you can't talk them out of charging for that when you buy skis.