I use canvas mailing bags to send out books from the library to patrons, they have metered mail on the address labels, they weigh around 2-3 pounds and meet the postal guide lines.

Can these be put in the blue box ?

My understanding is that anything can be put in the blue box IF it has metered or electronic postage on it, but NOT if it has stamps.

Thats what I figured but I had a library patron call me saying her bags had been returned to her and she was told they were not allowed to be put in the blue boxes.. I went and contacted the USPS so hopefully they can explain why it happened.  weird since I put all my PBS stuff in the blue boxes. I might try out the bags myself if my work will let me that is.

It's supposed to be ok if it has traceable postage on them.  THey can't trace a package with stamps back to the purchaser.  But they can trace a package with electronic postage on it back to the purchaser.  It could be that someone on the postal end messed up.  I've seen a lot of complaints on here about printed postage boxes or boxes where they went to the counter and bought postage being returned for no reason.