Base charges on actual transaction cost, not list price.
I give freebies and other substantial discounts to my customers. It appears, though, that the transaction fee is based on the list price rather than the actual cost of the transaction. If I give away a $100 digital item as an incentive to sign up, it costs me $1 (1%) for each free product I give away. I'd like to see you either clarify that in your marketing materials, or actually base the transaction fee on the transaction total rather than the list.