Ask HN: Implementing reward points between unrelated merchants.

Say I want to create an app in which users can earn points for purchases at different, unrelated coffee shops in town. What would merchants need to get on board? It seems like a separate scanner or special software would be necessary, or maybe the ability to print scannable codes on receipts.

I'd like to build something similar. If anyone can point me toward resources or products I can look into that help make this possible it's much appreciated.