Coupon codes: I need to track which coupon is used.

Using eCart. Coupons work great. No problems with the math.

New client request: To capture which coupon is being used with which purchase. I assume, first add another field to the db. And redo the order results so client can see which coupon code goes with which order.

But then, I'm stumped. I need to tie the entry to the database. Since the cart.php shows in bindings, after I add a coupon field to the database, I assuming I can tie it but then what happens to the math?

I don't want to break the existing cart by doing this wrong. Does this need a premier ticket?

UPDATED: client just added a request to do this with PowerStore also, on two sites.