You can reprint the UPS Shipping label from your History page:
You may only reprint the shipping label if it has been less than 24 hours since you generated the label.
I had the same problem. Make sure that you are bringing up the Details for the transaction in which the person paid you, not the transaction where you paid for the UPS label. That is not quite clear on the PayPal Help page. There is a Reprint Label option at the bottom of the Details page related to the original purchase. I hope that helps!