That usually means that you accidentally specified UPS shipping when you listed the item (do a sell similar on that item and see what the shipping section shows).
(I looked at you recent sales, none show UPS shipping even as an alternative, and I'm a bit confused on how buyer could even select and pay for PM if it were the accidental UPS-as-only-shipping-option scenario that traditionally forced the UPS only label thing you are seeing)
You can print a USPS label for the sale through the PayPal* transaction directly through your PayPal account (just hit the "Print Shipping Label" button on the PayPal transaction, or can print a label manually through PayPal shipping
https://www.paypal.com/us/cgi-bin/webscr?cmd=_ship-now*with PayPal labels you get standard USPS Commercial rates on Priority Mail, same as eBay labels. Only a problem if you are Top Rated seller entitled to USPS Comm Plus PM rates.
