You don't actually need the invoice.
Just use the Paypal Send Money service.
You have his PP address/ email in the original transaction.
You can Message him through eBay or through that email to let him know what's happening.
I know this means you need a PP account yourself, but it is a very useful thing for buying online. I use it to pay for convention tickets, send gifts to family, buy on other sites, purchase subscriptions...
PP is a very secure way of paying online without the seller knowing your financial information.
And your have six months from payment for the resolution of any problems.