You won't get any answer from the Upwork moderators - just useless B.S. answers. Here's the simple answer: Paypal will automatically use your Paypal balance before they hit your credit card. That's all there is to it.
Did any client ever successfully use their Paypal BALANCE to pay freelancers? I don't really want to hear from moderators on this issue, since they have been blatantly ignoring the main question on this thread.