Google Forms PayPal Express Integration

I am doing work for a non profit with 0 budget for IT. They need to allow users to enter information on a Google Form and then collect payment. I have done a lot of research on the topic and currently appears you can only add a hyperlink on the Google Forms to link to PayPal. However, I was wondering if there was some way to link the PayPal Express Checkout javascript(https://developer.paypal.com/docs/integration/direct/express-checkout/integration-jsv4/add-paypal-button/) into the Google Form so it calls back with a transaction id and status. Then I would be able to somehow insert those values into the Google Form. Any thoughts on how to integrate would be helpful. I am a developer but not familiar with Google Forms.

I recently wrestled with this exact question and ended up building a G Suite addon to make this easier for others. Coincidentally I also do volunteer IT for a small nonprofit and we wanted to use PayPal with our signup form (there's pricing for the addon but just ping me if you're a small nonprofit).

Here's what I learned while building it:

There's no way for Apps Script to modify the form once it's shown to the user, or to use Apps Script to respond to user input
There's no way to redirect the user automatically to PayPal after a form is submitted

That means if you want to send someone to PayPal, you've got to use an ordinary link. You can do this within the form by adding the URL to a question description, or you can add the URL to the text shown once someone has submitted the form. For PayPal, you've got two options for this kind of link:

PayPal buttons
Express checkout

PayPal buttons are a static link managed by PayPal. They don't require any coding - just go here and create a button. Then you need to get the raw text link for the button, which PayPal calls the "email" version. You can insert that into your form directly and tell users to access it. The one problem is there's no real way to get the transaction ID. You could try correlating the form submit time with the payment time, or the payer email with the form submit email. It's possible that neither of them will match up and you'll have to do it manually.

Express checkout requires you to dynamically create a new link for each payment by calling the PayPal API. That means your link needs to open a page that then generates an express checkout URL and redirects the user to it. You can do this using a web app in Google Apps Script using a doGet() trigger, or you can create your backend on your server in any language.

If you can run your own server somewhere, I recommend that (plenty of PaaS services have a free tier). It's much easier to test and debug things when you aren't using Apps Script. I used the PayPal Node SDK which works great despite being unmaintained. (Express checkout is "deprecated" by PayPal, but I'll bet it's not going away anytime soon). Their example will get you most of the way there. When the user arrives, generate a payment link and redirect them. When they finish they'll return to your server, and you can display whatever you want. For example, you can ask them to copy the transaction ID and paste it into your form.

Finally, when your form is submitted an onSubmit() trigger can be set up. There are actually two kinds of onSubmit() triggers for forms - one for the form itself, and a second for the spreadsheet linked to the form. You can register a trigger to do extra processing (e.g. look up the transaction ID), but you can't modify the response in the trigger. You can however modify the spreadsheet where the trigger gets sent, which for most cases is equivalent. For example, you could add a column to the spreadsheet with a link to the PayPal transaction based on the transaction code.