I have a customer who sells books. She also sells greeting cards. She wants her shoppers to be able to add on greeting cards to their book order for no additional shipping charge. If shoppers purchase the cards alone, they will be charged shipping, but a max of $6 for unlimited cards.

I tried to do a handling charge of $6 and mark the cards free shipping as that would solve it, but the handling charge is not calculated with free shipping.

I need to figure this out. Your help is appreciated!