I don't know about most other tweens, but mine have been big lately on getting points or credits for various games; online game credits, video games, etc. I generally prefer they purchase their own pre-paid card when we go out. Sometimes I'll let them hand me cash and I'll pay for it with paypal. I've ended up with bags of change this way, so I've had to be cautious about that.
My son is particularly good at saving for the things he wants. He has saved for various computer games as well as his own gaming system. He recently found an online game that he enjoys, so I helped him out. He gave me cash, I paid for him.
My daughter, on the other hand, loves the idea of credit. Let her purchase it now using my money and she'll work it off later. I've had to put a stop to that as she tends to forget she owes me. Cruel? In her opinion, yes. In mine? Better to learn not to over reach just because something sounds good. She recently asked for something as well, but didn't have the cash on hand for it. I took a look, discovered it could wait until Christmas, and said no. So, she'll either earn it in the mean time, or put it on her wish list.
Personally, I think we're doing well by our kids this way. Do they have lots of stuff? Definitely. Could they do without it? Well, they'd say no, but in reality yes. They are, though, kids. So I'll give them the chance to save their money and then spend it the way they want to.