Tracking is not the only proof PayPal accepts. Well, it is for mailed items, but for service/intangibles/local pickups/digital downloads they look at the messages and the seller can forward a copy of the email with headers to PayPal that contained the link and PayPal will look at other supporting information.
"Retain proof of transactions.
You’ll need to respond to all claims. If a customer opens an Item Not Received or Significantly Not as Described claim for an intangible or a service, we’ll need evidence that the buyer received the goods or service, or benefited from the transaction, and that it was as you described it to be.
Supply appropriate documentation.
Provide as much relevant evidence as you can to support your response to claims. The more the better. For example, for sales of digital downloads you might provide proof that you sent the buyer a link and that the buyer accessed the download including dates and times."
https://www.paypal.com/uk/webapps/mpp/merchant-intangibles-update
