Add Loyalty System, which means when a customer buys a product he should be added some points which can be redeemed in next purchase or later by converting them into money. Here add Card system means membership card system/ phone number, where a customer links with card and when on POS screen once phone number entered it will show the Membership ID, Points with them on Screen. And a give an option for seller as he can give chance for full redeem or on sis, so that he can choose the option for customers as a default.
And also add an option for giving automatic points for new customer, festival points, purchase cutoff times (like after every 10 purchases some points should be added automatically)

Membership card Barcode/ Manual entry of number on Card.