A book I mailed with DC is officially lost.  I contacted USPS a week ago and they have not returned my call.  I had already contacted the requestor and he hasn't received it.  I got the email this morning from PBS that the book is lost and I sent them the message (as they requested) that the book was never scanned (even though I did put in a blue box at USPS).  I would like to now call the PO where I had initially mailed it, since it was never scanned there (and normally all the books I mail are scanned that same day, so I would assume it is part of that PO protocol).  But now I can't find the Details of the trade anywhere.  It is no longer under My Account etc so I can't find that DC code that I will need when I call the PO.  I am sure it is obvious and I just can't see it.  Please help!  Thanks so much :)

look in your Transaction Archive

Once the system marks a book as "Lost in the Mail" it closes the active transaction and moves it to your Transaction Archive, as Christy mentioned. 

Get to your Transaction Archive page by clicking on My Account at the top of any page and then clicking on Transaction Archive link from the bottom of the dropdown menu that appears.

As the others have said, go to your transaction archive.  If you click on the details link, the DC # should/will be there.  You can take that number to the post office for them to look up in there system.  It's what I had to do once.