How to send out a voucher when registering?

Hi everyone,

Has anyone done a mailing that would send out a voucher code dynamically upon registering?  The scenario is that customers can register to receive a voucher for a discount.  There is a limited number of coupons and each person that requests them would need to receive a unique code.  There are a set number of vouchers so once they have been sent out then no further codes would be available. Once they register an email is sent with their unique code.  I've thought of picklists, using a cdo but come up with an issue on how to ensure a code