State laws will vary on how financing costs are handled. Here are a couple of recommendations:
1) You can use a blended average cost of capital. Boost all prices by a fixed amount, say 6%, to cover your blended cost of financing. Some customers pay more and some less.
2) We believe that the wording on the app allows you to offer discounts, your list price is set at the highest for all the rates that you use. But this is not a legal opinion, you should consult your attorney.