Freddie here from the Shopify Support team, happy to help.
If you're needing to track the thank you page URL that appears after a customer has completed their purchase on your store, we recommend you to use
checkout/thank_you, because the checkout page URL itself is not static.
Our checkout generates tokens for every session as a string in the URL. A customer checking out on your store will see something like:
Seeing as this link does change every time, if you are wanting to track thank you page visits, using
/checkout/thank_you should work just fine.
If that doesn't work, it may be best to contact PayPal support to confirm with them what the appropriate link is that they need.
Let me know if this helps!
Shopify Support - email@example.com