I've been reading on their Instant Payment Notification, but it seems very much overkill for my simple request. I'm wondering if their newer Webhooks is an option, but I'm not understanding their documentation on it.
There's also a huge lack of any sort of PayPal Webhook examples or tutorials other than PayPal's documentation. If Webhooks is an option to send a user inputted value to a PHP script on my server after they've completed their PayPal transaction, how would I do it?
You can use a custom variable
<input name="custom" value="some custom value" type="hidden">
Custom Variable is given as:
Optional pass-through variable that will never be presented to your customer. Can be used to track inventory.
This variable is returned in the callback after the transaction.
You can find sample working forms and variables here: https://www.paypal.com/cgi-bin/webscr?cmd=p/pdn/howto_checkout-outside