I was staying over at this girl's place and she said that I should buy her a CO2 cartridge for her water carbonator. She said she would pay me back. Whatever, we fuck, I don't care so I just buy it when I'm at the grocery store. Mistake?

Next time I see her I give her the cartridge and she says, "Awesome. Thanks!" and goes on whatever she was doing. I say, "Um... What about the $20?" She says, "Oh come on it's only $20..." "Did you not say you were going to pay me?" "Don't be so cheap."

I fucked her and haven't contacted her since. She promised but she didn't deliver and tried to guilt me out of collecting. Usually I don't even care about the money. I just see it that I paid $20 to find out that this person is a piece of shit. Red flag -> next.

Anyone do anything differently here?