how do you "take cash" over the Internet?
How it works is you purchase a product online and it gives you a barcode that can be scanned at any major convenience store. You go to the store, scan the code, hand over cash, and the content you bought is instantly unlocked once the payment is confirmed.
Cash handling isn't really the problem with this suggestion
I don't think Valve could feasibly implement this at their scale - especially if this method was the _only_ way to acquire the games in question.