I am in Minnesota but selling an item to a person in another state. If this isn't "interstate commerce," I don't know what is. The buyer wishes to make payments on the item (in sort of a "layaway" arrangement), and I will ship the item once the payments have all been made. Is there a maximum late fee I can charge for a late payment? There will be no interest charges in the payment agreement other than this possible late fee, if that can be considered "interest." If there is a maximum late fee, what is it?