Since they do get a Square email receipt, it seems to me you could just carry one of these for the few who want paper. Top of the form can be customized if you wish by custom order or with a cheap "return address" type sticker. No power supply needed. Ink is a pen. Available off the shelf from Staples, Costco, OfficeMax, Office Depot, etc.
You could hand write the Square transaction ID on the form.
Receipt Book, 50 for less than $4.00