Transporting children to non custodial home

my ex and i have been separated a little over a year. he moved in with a girlfriend 20 miles from here at that time. we have a verbal agreement that he picks up the kids every other weekend and has them for 2 nights. lately, he has been expecting me to pick up the girls on sunday evenings, claiming i should do it because he pays child support. i am not the one that decided to move so far out, and i don’t always have enough money for gas to drive out there. i usually fill up my tank when i get paid and i work close to home, so it can last 2 weeks if i don’t have to drive to knightdale. i already agreed to let him off $125 on child support because i did not want to wait 4 months for a court date.
who usually pays for transportation in this situation?

There is also no “usual” for payment of transportation if there is no order stating who transports the children for custody exchanges.