Paypal's "Help Center" (not always helpful)
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https://www.paypal.com/us/cgi-bin/webscr?cmd=_help-ext&eloc=1647&loc=1640&unique_id=56544&source_page=_home&flow="
"Protection Policies - Seller Protection Policy - Tracking Shipments
Q: What methods of online tracking are acceptable?
A: The following options may be used:
* Parcelforce
* Royal Mail
* U.S. Postal Service
* UPS
* FedEx
* Airborne Express
* DHL
Note: Royal Mail does not officially provide online tracking (with proof of delivery) for the International Signed For service. In some instances however, delivery details may be available online. If these details map to the buyer's shipping address shown on the Transaction Details page, PayPal may consider it as sufficient proof of delivery. However, since availability of delivery details cannot be predicted, sellers may not be able to rely on this method for proof of delivery in order to be covered by PayPal's Seller Protection Policy."
More on the Seller Protection Policy (which doesn't always protect sellers) is here:
https://www.paypal.com/us/cgi-bin/webscr?cmd=_help-ext&eloc=0&loc=1609&source_page=_home&flow=