Contact the buyer and explain the problem. Politely let him know that it was his information that led to the problem.
Ask if he would prefer to cancel the transaction or to pay the additional shipping charges.
Then wait.
Remember that while you can refund less than his full original payment, that he will be able to leave feedback and Detailed Seller Ratings.
Let him make the decision.
With luck the worst problem will be the postage you are out.
It's business, not personal. Always build a little wiggle room into your pricing to allow for these occasional problems.
Politely. - Captain Malcolm Reynolds
