The customer may not realize that he is obligated to pay.
If you have sent him an invoice, your next step is to open an Unpaid Item Dispute in the Customer Support-> Resolution Centre. You have to wait four days for this, unfortunately.
If he is just a slow payer he will probably apologize and send money. Leave him polite feedback to encourage the same.
If he does not respond, close the Dispute giving him a Strike, which will make it harder for him to bid and buy on eBay in future. You will get your FVF back. He will not be allowed to leave feedback.
