Walmart has already done the thing that you are describing; they have had Walmart Pay for some time (https://www.walmart.com/cp/walmart-pay/3205993 https://www.

Walmart has already done the thing that you are describing; they have had Walmart Pay for some time (https://www.walmart.com/cp/walmart-pay/3205993 https://www.walmart.com/cp/walmart-pay/3205993). Starbucks also pushes their app and paying via app, which has the side-effect of bundling 5 $5 transactions into one $25 transaction (a "card reload"), thus reducing their fee overhead.