QR code validation.
Every mobile device (phone or tablet) with a camera and QR scanner can be used as validation system. At validation, the cashier scans a QR Code on the clients mobile phone or print during checkout. Scanning the QR Code generates a page to be displayed on the cashier mobile device, that releases the coupon validation. At the same time, the client's mobile phone shows the 'processing' of the validation, and when the offer has been redeemed, the coupon shows a 'redeemed' mark, and it will no longer be valid.