Reduce rental car tank refill charges

Car rental companies charge ridiculously high prices per-gallon if the customer doesn't return the car full tank. Makes profit, sure, but also makes customers unhappy. Why not give small discounts (say $5-$10) to all customers who return the car full tank and charge a progressive flat fee (say $1-$10) on empty tanks based on the amount that has to be refilled? Customers on empty tanks will be penalized less than if they had to pay on a per-gallon basis and everyone gets the incentive to return the car full tank. The discount can also help lure more customers to the company.

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