I just received a book from TX and the member used DC and PBS postage.   On the front PBS label was another 1-1/2 x 1-1/2 sticker.  It had a barcode with my zip code and then under DAL followed by a 10 digit number.  It was placed over the senders name and Request ID. 

Does the PO have their own scanning system in addition to the DC that was purchased?   I am just curious - the book arrived safely.  I thought at first it was a theft sticker but it peeled off.  so now it looks like the barcode you see when you buy towels for example (that size).   Anyway - any ideas why?

I've seen labels like that on my received packages.  My mom, who works for the USPS, says that they might be labels that help the machines sort the packages.  She wasn't sure, though.

I'm all for anything that helps me get my books sooner, so if that's what it does, then yay.  :)

I get them often too. Usually stuck over my address. I never looked closely at one of them but do remember that my zip code is at the bottom of the sticker, below the bar code. Maybe the bulk sorting center applies them? 

I get those frequently,  also assuming a sorting tool.

I've gotten a lot of books with those. Mostly they've been books from bookmooch with handwritten labels. But I've gotten a lot of PBS books with them. I also assumed it was just a post office sorting thing.

I just rec'd another from TX - maybe it is because the sorting centers are so large???  Thanks for the response, I am naturally curious about meaningless stuff :)

They may be meaningless to us but I bet they mean a lot to the PO machines!!  I get those on all kinds of packages.  I always thought it was a routing thing.