How can I change the invoice after eBay has sent it out

[bantwo]

I've sold 2 items to the same person with combined postage. EBay had already sent out the invoice before I could change it...

The buyer can't pay me. What can I do now?

[arkwebus]

Open a non-payer case in the resolution centre [link below] and close after 4 days.

I can see no reason why a buyer "can't" pay you.  If you mean that you want to send and make a postage reduction you can refund a portion after sending through Paypal

@bantwo