UPS labels are supposed to only be charged once scanned.
You can test this by going to My Ebay on the Desktop browser, then going to the ORDERS drop down, selecting SHIPPING LABELS, and finding your transaction. To the left of your transaction will be an ACTIONS drop down. There should be an option that says CANCEL LABEL. When you click this, if it can be cancelled it will cancel it. But I think with UPS it will say no need to cancel, it won't be charged if not scanned.
One issue you might have if it is a US label is that it might automatically request import/brokerage fees on the tracking page. I've printed a label one afternoon and a day later before it is even handed to UPS the package is assessed brokerage fees. You would have to contact UPS and ask if you will be on the hook for these for a label that is never used. Usually, if a buyer doesn't pay these, UPS back charges them to your eBay account.
UPS tracking also has a feature where buyers can tell it to delivery the package to their local UPS store for pickup. I would check with UPS to be sure that this would work, but if their address is a PO Box and they selected this option on the tracking page, it's possible that the address being a PO Box would be irrelevant since it's not being delivered there anyways.
