- Does the digital counter has some means of communicating its value electronically in stead of only for human eyes? If yes, what ways are possible.
- If that can't be done, you might have to tap the system that sends the count to that digital counter to show. What system is that (a point of sale system?) and how would you get the current count out if it electronically.

By the way, an Arduino or a phone or a tablet or even a small laptop certainly fits in even a small shop. So it's not clear at all why you can't use it. Can you explain?