The in-store cards are a standard method of payment. Just hand the card to the check-out assistant or if you are using a self-service checkout swipe your card to pay for your goods. Always ask for the card to be given back to you as these store cards are re-loadable.
E-vouchers can be used in a similar way and provide a unique code that contains details of the amount of cash that can be spent. Please print out the e-code from the email you receive and take this to the store, some stores will also accept the e-Voucher on your mobile device.