How about this: you order $x,xxx to $xx,xxx or more prepaid oil from a supplier in your area for this year, affiliate gets commission. Lock in the price for a year; if the price of oil goes below your lock-in rate before delivery, you also get a reduction; affiliate commission stays the same, however, and was already paid. Also, to make it a real affiliate partner program, affiliate receives commission on all your future orders as long as you remain a customer, and second tier commissions, too.