I'd be hesitant to give a gas station app ACH access to my checking account. I guess you could open a second account and just transfer enough there to cover your purchases but that's a bit of a nuisance.

I'd be hesitant to give any app any sort of access to my checking account. I would only do so with a second account with just enough there to cover my purchase. While it is a nuisance, the phone is simply too large of an attack vector to consider anything else.