artists_beware | Second Round Shipping Query

Hello, A_B, I come to you with a question, and your opinions are greatly appreciated.

I've recently took on a bunch of badge orders that were mailed out to their various buyers back on March 13-14th. Almost all of them reached their destinations, or so I thought. Today, one of them came back in the mail, labelled by the USPS as "Return to Sender, Undeliverable. Unable to Forward." I found this strange, because the customer confirmed the same address with me twice, and the address I was provided matched the confirmed address on their PayPal info.

Now, I have attempted to make contact with the buyer, and though I have yet to hear any response, I was wondering about the issue of shipping it out again, and the costs involved the second time. Being as that I did ship it originally using the money provided by the client, am I out of bounds to ask the client to pay for another amount of shipping charges? We're not talking a lot, the bubble mailer is still able to be re-used as it's still in good condition, as long as I can remove the "return to sender" labels cleanly.

I was thinking as I charge $5 to ship out small, real-media artwork, which includes the costs of a padded mailer to protect the work as well as the actual shipping costs, that as long as I could re-use the mailer, that $2 would be reasonable. But what I'm curious to know is, is it considered bad form to ask the client to pay more shipping costs, when I have clearly fulfilled my end of the sale by completing the client's order and shipping it once already? Am I to be held responsible for the failure of the package to be delivered correctly?

Let me state again for the record that as far as I can tell, the address I used on the label is exactly the same as the both the address provided by the client through e-mail and the address confirmed in their PayPal info. I cannot find where any mistake was made on my part, and out of several packages mailed on that date to various clients around the country, this is the only one that came back.

Thanks for taking the time to read and respond.

EDIT: Thanks for all the input! While I could technically have asked for extra compensation, I decided to take the good PR route and just eat the costs on shipping again. I did confirm again that the address I used was correct, so I'm re-sending the package with the hopes that this time it'll get through. If it comes back a second time, I'll address the shipping charges issue at that time. Thanks, everyone!