Good deed or sucker

I stopped to get some dinner on the way home. Where I was picking up dinner is a popular district in town, and I had to park a block away and walk through a gas station lot on my way to get Jimmy John's. On my way there, I noticed two girls pumping gas (they are doing all kinds of construction, and I was wondering how a car actually got into the gas station). After getting my sandwiches, I returned the same way, and was surprised to see these same two girls sitting outside the gas station, obviously distraught. It turns out they only had a check, and the gas station does not accept checks (although this apparently is not marked), so I charged their gas, and took the check from the girl. I can't imagine that a young college-age girl would go to all that effort to scam me out of $15, so I have to trust I did a good deed.