A couple of things suggestions:
EDIT: 1) Found the problem -- conflict with a mod I made.
2) It would be ideal to be able to assign more than one subscription ID to a coupon. You could code this pretty easily using "explode" for commas.
3) When you assign a coupon code, the code is changed to lower case ($coupon_name = strtolower($coupon_name)

. That isn't good for me because I'd like to use all-caps in my codes. I can just comment out the line, but you might want to make it optional or something.
Thanks again for this mod! It's something I've been waiting for forever.