you have to cancel over the phone and the CSR will give you a voucher number. I do not recall getting an email with the number.

After using the voucher for less than its value I called to get the number but I do recall getting an email with the CREDIT number.

I suggest calling in both cases just to make sure I actually have the numbers and make sure you fully understand the process.

just beware for some reason they cannot be very explicit in explaining how to do this. you have to read between the lines a little.