The mobile card readers I've seen don't look much different from the fixed-in-place ones at store checkouts. Just a little bigger to accommodate a battery and the wireless "stuff," and maybe tending more to rectangular than square so they fit better in someone's hand.

Like most things, they're getting smaller all the time. Waiters have them in their apron pockets at a lot of restaurants, now. A windshield repair place has probably been using it for a long time and so probably has a larger one, more the size of the things you sign on when you get a UPS or FedEx parcel.

So 7rider, you did your good deed for the day!