Was the message also in your Ebay inbox, or just sent to your regular email? Red flags are up for me.

Do not pay anything extra. Not your fault they haven't calculated postage correctly. However, this sounds more like they're just trying to get extra money off-site as they quickly contacted you after the sale with the request. If you agreed to this, they would probably want to send a Paypal invoice for this additional "postage", as there is no way to get more money from the buyer once the transaction is made. 

Can you provide the item number so we can take a look at the seller?

You do not need to report, but you can message the seller that you're not paying more than what is asked and they either need to send the item or cancel and refund.