Discount code on invoice

Hi. We have several discount codes in use. The trouble is we don't know who used what code.

For example, we have a code that allows for full membership, but they have to pay in person when attending a class. We created a coupon PAYLATER. We also have people who will have free memebership so we created a coupon "iwacompmember" which has also been used, but we have no way of knowing who used what.

How can we include the member code either in the invoice or in the subscriptions so we can export them?

Thanks!