We recently implemented an open source point-of-sale at the Film Society called uniCenta oPOS. Totally free and it took about 30 minutes to set up two computers and connect them to a central database. It's the best free one out there, after several months of trying different systems. Our volunteers, young and old, come out to work for an hour at a time, once or twice a month, so ease-of-use was very important. The system required about 10 minutes of training, after which everyone so far has picked it up pretty quickly. I highly recommend it if you're ready to move off paper at your gift shop, school store, or ticket booth.
It's touchscreen based so you can do it all without touching a keyboard. It integrates pole displays, receipt
printers, and cash drawers. We're
running it on 4-year-old donated laptops with very low system specs, and it's zippy and responsive. It's Java, so it will run on Windows, Mac, or Linux.
I'm working to get Internet access at our venue, so we're not taking advantage of the credit card integration and I can't speak to it yet. And our merchandise inventory is pretty small (fewer than 50 different items) so we don't use its barcode scanning features, though I've read up on some small convenience stores using it very successfully.
You can read more about it from my blog here:
http://nicholaiburton.com/blog/2012/finally-a-decent-open-source-point-of-sale-system. I'm also a PATH volunteer, so post replies here with questions, or create a ticket if you're interested in implementing it at your nonprofit and I can help you get started.