150465 Oscommerce Voucher Mod

Hi, We use OsCommerce as our shopping cart and would like a mod added to it. We'd very much like to offer customers a £5 voucher for their next order, once they've placed an order. This can be via email, or a voucher code displayed at the end of the checkout process - whatever is easier. Lastly, we'd like the facility, doesn't have to be too tidy, but so we can turn it on and off. Many thanks, Ben