Purchase store credit

How do I allow customers to purchase store credit and have that credit immediately available? I need to do this as the payment system we are using only allows customers to make an individual purchase of up to $600 in a single purchase. They therefore need to be able to make a purchase of store credit or I suppose a gift voucher that can be immediately available to them to combine with the purchase of the item, as long as the gift voucher can be automatically generated and emailed to the customer.
Any help would be much appreciated.