You could use "show invoice details" to pull details when the user enters the invoice number. You can display this however you want.
The response there will include recipient_view_url which can be attached to a link/button so the person can click to load it and submit their payment on the PayPal Invoice directly. That will take care of updating the PayPal status accordingly.
Then you can use invoicing webhooks to update your own system, send out custom branded emails, or anything else you might want to automate based on PayPal transactions.
Angell EYE - www.angelleye.com PayPal Partner and Certified Developer - Kudos are Greatly Appreciated!