This happened to me this week.  I mailed out a bunch of packages over the weekend.  Most of them were received.  One came back to me with the address label torn off (it was under packing tape) and some of the stamps were gone, too.  You can see a piece of one of the stamps that was removed and the ones still on the packaged have a postmark on them.

That package came back to me on Wednesday.  What else do I do?  Is there any hope of sending the pkg out from the PO and not having to pay for the postage on there that has a postmark?  And, I suppose, that I would have to go to the PO that handles the blue box pickups (where I mailed it) and not to the more convient one?

I'm ticked that this package came back, but I do know I had everything needed to mail it was there when it left my house. 

Sorry, I'm not clear on whether you mailed it from a blue box or from the post office counter (handed it to a worker).

Was it more than 13 ounces?

I'd take it back to the post office and ask THEM what you should do about it.       Looks to me like a mistake on their part.

It was way under 13oz -- and it was mailed from the blue box across the street from my apartment. 

I would go to the PO(preferably the one that services the blue box) and ask them what happened and if you can get your postage back.  You need to PM the requestor and let them know what happened and that it might be late.