Is that screen changing swipe due to slow machine Or is it an animation? Looks bad UI/UX if it's animation, the calculator doesn't look snappy due to it. Building such a hardware must be the most difficult feat, although an app on mobile would work with the same use case but there is no beating a calculator for the speed and shop keepers being very proficient at it. A calculator that does the ledger sounds good. Best of luck.

Disclosure - I worked on this :)