when i owned a shop that sold gasoline i would always ask a customer if they wanted a top off. i would keep that slip separate. when the customer picked up their car the slips would be added together. any discount on the order wouldn't effect the gas. my recap sheets would have been a nightmare otherwise. to this day if i have to fuel up a car or truck while i'm servicing them the gas slip is an additional invoice even if there is no discount involved.
never argue with a fool, they'll drag you down to their level and beat you with experience