Depends on your exact requirements (ie, are 5 or 15 minute delays acceptable) but you can get started super quickly using Zapier
https://zapier.com/zapbook/firebase/paypal/
You would setup the zap using your secret firebase and paypal keys, setting the zap to update a property on the user's profile (be sure to use Firebase security rules to limit write access to that property) which says they have paid.