You cannot do this through eBay now that the buyer has paid you.
They can send the extra payment by whichever method suits you both best, that is, by cheque, bank transfer, or if they have a Paypal account a PayPal Friends and Family payment.
Or you can send them a PayPal invoice, they don't need a PP account for this , and can pay you via various card payment options.
If PayPal is used, email addresses need to be exchanged, this can sometimes be found with the order details >> Show Contact Info. If not and make it very, very clear in your eBay messages, as eBay do read these, the reason for these exchanges. Make it clear it's not to be used for an off eBay sale.
