Here's one of those questions where I wish I had gotten the patron's contact information, because not only did I find out after the fact that I gave him bad information, but I also found a good answer later on, too.
One afternoon someone from the Circ Desk called down to Reference. She said there was a patron who wanted to donate money to an online charity, but all he had was cash and he wanted to know if we could help him.
I immediately started to mental spin through all the ways I knew of to pay for something online, and none of them originated with an actual fistful of dollars. The best idea I could come up with on the spot was some kind of prepaid credit card, which is not something the library offers (but I know there are lots out there). However, right across the street from us is a bank, so I just recommended he go over and check with them, optimistically hoping a bank product would be some kind of cash card he could use.
After work that day curiosity got the better of me, so I walked over to the bank and asked them this question myself - and I was surprised that the teller's answer was "no." She said the closest thing they had were regular debit cards, but those are tied to an account. She said they do get requests for something like a prepaid cash card, because lots of bank customers don't want to expose their account information online at all - but they just didn't offer any kind of "internet gift card."
That phrase made some connection for me, and got me wondering if PayPal offered a gift card, much like I see eBay and Amazon gift cards for sale at cash registers. Sure enough, they do! I also found Green Dot MoneyPak, CoinStar, PayNearMe.com, and CashPayment.com.
Drat that I didn't think of this when the patron was in the library. Of course, he'd still have to find a nearby participating retail location, and the charity would have to accept one of these, but at least there is a way to turn cash into an internet-friendly form.