You can use a previously purchased Gift Voucher as your Payment method.
To do that you will need to use the Gift Voucher Unique code.
To find out how to see your Gift Voucher Unique code for payment click here.
When you create a payment (any type of payment) you can choose the Gift voucher option from the dropdown as your chosen payment method.
To find out more information about adding Payments click here.
Next add in the Unique Gift Voucher code.
If the voucher is not fully redeemed, the remaining money can be used for subsequent payments against services in the future until the full value of the Gift voucher is used up.