I recently wanted this exact thing, and (since I'm an engineer) built something to make it happen. You can use this tool to add a payment question to your google forms with a unique link. When the buyer follows that link and makes a payment, they'll get a receipt code that includes the PayPal transaction ID. They then have to copy a valid receipt code into your form before they can submit. To my knowledge mine is the only solution of its kind for Google Forms, particularly how it validates responses so you don't have to try to match up payments later. If you give it a try let me know what you think!
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