Well, since it was your deal and not mine, I get a chuckle just thinking about their "logic".  Yes, that first fillup was gas used for the trip.  You should have at least been able to convince them that the last $20 was not for gas used on the trip and saved $10. lol

Sometimes when logic fails, a person just has to smile and say "OK".  Good deeds sometimes come at a higher than expected price.  That "extra mile" can be a long one at times but the reward is worth every cent even if you don't see it; if we need to see it we may be doing it for the wrong reasons.  You did the right thing in questioning their logic and in covering it all.  Thx!

Those smiles are priceless.