AbleCommerce Shopping Cart Software

I want to set up the following situation in AC7:

If customer buys product A, B, or C they receive product D at a 100% discount (shipping/applicable sales tax still in effect). Products A/B/C are all kits.

I just looked at coupons and it doesn't seem that I can link the purchase of specific products with each other with a coupon code. Is there a good way to do this other than adding product D to the kits with a price of $0.00?

What about upon checkout in CheckingOut method iterate over basket contents if it contains the desired products simply add product d and also apply corresponding coupon on basket as well through code.

This will need some more thinking. If it's to be achieved using a coupon, it'll need a bit of modification.