I'd be more inclined to believe that there is a formula, something like this, Humminbuzzard takes a chance that not everyone will send in the mail in coupon, some will forget and go past the end date, some will loose the coupon, and some are just too lazy, so the lower the amount they have to redeem, the more they "save" or less they loose on the offer. Then again, a percentage of the cost of the "free" transducer is already built into the price of the unit.