I just implemented this new PayPal code into my Google Forms add-on named Emails Director Gold.
You'd need to install the add-on.
The store listing is at:
That add-on doesn't have a free tier. It has a free trial period.
The PayPal code that I've provided doesn't include the code that I use to determine the trial period or whether the user is paid.
I don't have many paid users, so I just save customer information to a Google Sheet.
The customer information saved to a Google Sheet is obviously done with server side code, so as soon as the payment has been
accepted, the client side code makes a call to the server.
The PayPal code provided is intended to be stripped down and generic, it's not the exact code that I use.
So, if you need code to filter out the paid and unpaid features, or the trial period,
this code doesn't provide that.
I don't have any sample code for that.
Do you have an app that you'd like to accept payments/donations for?
If you are accepting donations, then you don't really need the server side integration, and it would be much simpler.