Why do you need to create a credit voucher? You want to increase your revenue or boost your sales during a special occasion, so you offer a very special deal during the period, and this credit voucher is equivalent to CASH. 

Your customer can use this credit voucher to redeem any products from your store without informing you to create vouchers every time she needs to redeem the products. 

This saves you time and effort in maintaining a copy of the information in excel or sheet for your customer's credit balances!