Dynamically sending a voucher code

Hi everyone,

We have a process where a customer can request a voucher code for an event. Once they submit the form we send an email to them with the voucher code. They can then use this code when they register to get a discount. The voucher codes are pre-created and limited to x amount of codes. We need a process to be able to automatically send an email with the voucher code and not use the same code twice.  I've thought of picklists, using a cdo but not sure how to dynamically pick the next value in the