E-book readers in their actual format are too small for the use you are planning. It would be a nightmare for a shop keeper to control. Think first of the cash outlay for the acquisition of the devices, and second the theft threat!
Being a cabinetmaker I would take a different route. I would build a thick tabletop on a fixed table. This table would be bolted over a connection for power and ethernet. The top would be thick glass under which you'd find a touch screen of whatever type you can find with the cpu locked in the pedestal. This system would be foolproof and impervious to coffee spills. There is no e-ink screen today that is large enough for this use, but I don't think we'll have to wait long for their appearance on the market. When color is available, computer screens will be the next step.
If a big rich furniture company was willing to niche market this product, it could contract with an electronics firm to have custom screens built on a large scale, but that is a big
"IF"