You can only charge your buyer extra for postage if the buyer has agreed to this, for example asking for a more expensive postal service, after buying and paying.
If they have paid , nothing can be done through eBay now, they can pay you by cheque, bank transfer or a PayPal Friends and Family Payment, or you can send them a PayPal invoice for the amount.
If this was just an error in calculation you made for the postage, you cannot charge the buyer anymore money.
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 you need to ask the seller for this, 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.
@davidn4657
