Well, if you did this, you are not the first, and you probably won't be the last! The first thing you should do is let the requestor know what happened, by sending a Personal Message: click on the active transaction for the DVD on your My Account page to do this. Members are usually very understanding about such things--everyone makes mistakes! But if a package arrives postage-due without warning it can be irritating. Best to let the requestor know that this may happen.
All you can do after that is wait and see what happens.
Several things could happen from here:
The USPS may return your package to you.
If this happens, you can just put the postage on it and send it out!
If the DVD is returned to you, and it has been already been declared Lost in the Mail at SwapaDVD:
You should ask the requestor if she would like you to resend the DVD.
If she or he has already gotten a copy from another member, she will not want a second one
if she or he has not gotten a copy, you can resend the DVD,
When she or he gets it, she or he can mark it received from the Lost Transaction in his or her Transaction Archive
If she or he does not want the DVD, you should repost it.
The USPS may deliver the package to the requestor postage-due.
She or he may pay it, and contact you via PM to ask for reimbursement.
Or she or he may refuse the package, and it will come back to you
Occasionally the package will not be delivered OR returned.
We think this happens very rarely.
If this happens to your DVD, it will be unreceived by the system action date, and will be declared Lost in the Mail.
Sometimes (not often), the USPS does not even notice, and delivers the package (NOT postage-due).