AITA for refusing to buy drinks for my friends?

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Ok, so I (19M) am in college and in a friend group of about 10 people We are all under 21, which means that in the US we cannot buy alcohol. However, I have a fake ID that works at most of the convenience stores in the area and can easily buy beer. My friends, obviously, ask me to get drinks for them since I am the only one in the group that has a fake, and they always pay me back afterwards. None of that is a problem for me—I don't mind being the beer run guy, and as long as they pay me back, it's all good.

This is where I might be the asshole. I got laid off from my job recently, and money's a little tight right now. My friends asked me to get drinks for this weekend, and I said I couldn't unless they paid me up front because I simply don't have enough money to do it our of my own pocket. They were not happy with this. They claimed that it's much easier for them to pay me after, because I can show them the receipts and they know exactly how much they owe me. I said again that I knew they were good for it and that I never minded getting drinks and getting the money later, but this time I needed it up front or I wouldn't be able to do it. I said I would keep all the change and remember who it needed to go back to. They said no, that they would pay me after.

So I didn't get them drinks. I did show up to the party with a six pack for myself, which I could afford, and they said I was a complete jerk for getting drinks for myself and not for anyone else.

I feel like I was justified in my actions, considering there was physically not enough money in my bank account to get drinks for the entire group, and they refused to pay up front. However, the entire group is pissed at me now.

Am I the asshole?