If the buyer has not yet paid then you can simply send another invoice with the revised postage amounts.

As another poster has said, however, that may result in you receiving negative feedback or the person may reneg on payment.

If you do send a revised invoice I would suggest also sending an ebay message immediately, explaining that the first amount was a genuine mistake and that you would lose out if it was left as it was. Hopefully they understand and accept the explanation.

If the buyer has already paid the initial invoice amount then there is nothing you can do but accept it and take it on the chin as refusing to supply would almost certainly result in bad feedback and an angry customer.