How can I offer discounts on a main product without affecting the free add-on?

I currently sell a product, that consists of two parts. There is the main part, Part A and there is a secondary part, Part B, that comes in different sizes. Part A requires a Part B to be complete but Part B can be purchased on its own.

Right now I am using Assortion’s Add-on feature to make 1 Part B required and free with the purchase of Part A.

This current setup works fine except when discounts are used with the purchase of Part A. Using a discount code with Part A will make Part B have a cost instead of being free. I have set discounts to combine but it doesn’t seem to work.

I am looking for a way to make this work so that Part A can be purchased with a discount code without eliminating the 100% discount on the add-on Part B.

I am willing to use a different app if there is a better one for my needs.

Any help would be appreciated.