How can my customers store use gift vouchers?

After sending the voucher code to your customer's mail ... he can access any product equal to the amount of this voucher and enter the gift vouchers code

The following image is from one of the stores that enters the gift voucher code for the product when the order is completed:

After entering the code of gift voucher you can be pressed at send and then complete the order ... and then the product will be purchased through this voucher...