If the buyer has paid you you cannot add any postage now via eBay.
The postage cost agreed, the buyer can send to you via a cheque, bank transfer, or a PayPal Friends and Family payment. Or you can send them a PayPal invoice for the amount. Whichever suits you both best.
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.
@colinholford
