I used printable postage with DC when I mailed 5 books at the same time.  Dropped them in the blue box on my way to work and forgot about them.  They all went lost last week and when I looked, they hadn't been scanned at any point.....Well, got home today and I have all five of them back in my mailbox.  No note, no nothing. 

What do I do now?

I am not that familiar with the printable postage... but the first thing I would do is PM everyone you who was supposed to recieve the books and let them know what is going on.   Then (I am guessing it is USPS printable postage), bring the books to the PO to find out what the issue was.  Then I would remail the books right there at the PO.

Sorry you were having problems with mailing them... hope it all works out.

"Were they all over 13 ounces? If so, even though you can use printable postage and drop them in the box, someone at the PO may not have realized that."

That was my thought as well.  I am QUITE familiar with the USPS guidelines, but there was a day when I just got ahead of myself and put three books in my home mailbox, all over 13oz. and all with just stamps.  They were all returned to me in 2 days..a little note saying they were supposed to be taken IN to the PO...well..I felt like a goofball, but then I was quite angry because all of the stamps had been cancelled...I do not ship from the PO that is associated with my zip code because we really aren't THAT close to it..I always go to the village PO about 6 miles away..the gals in there had said the stamps should not have been cancelled..of course, it was too late to do anything seeing how I had already PRINTED postage and mailed them once again from my home..lol.