Depending on number of designs you could do it where he pays a "deposit" which would cover the cost of getting the transfers made.
Then you give him a good rate per piece as heat pressing a shirt every now and then MIGHT not be a big deal for you. Then again, it might very well be a big deal. Just price that service as you need.... all he can do is say "no". Take into account all of the time it will take doing the fulfillment part of it and the shipping expenses (packaging and time as well).
