I suppose you could shoot the same amount toward your credit card, a mid-month payment, whenever you use paypal. But that's still two transactions, far from easy and convenient.

And I suspect "easy and convenient" is one of the big things lacking in online busking. Even though Paypal is a LOT easier and faster than the prior alternatives. But even for people who like Paypal, it isn't as easy as pulling the change out of your pocket and dropping it in a hat or guitar case.