I got a package returned like that, too (3 days later). I went back to my post office and got my postage refunded, then went to a smaller post office and sent it as originally planned. I've been hearing a lot of complaints lately about mail being returned for silly reasons. I guess it depends on how much of a nitpicker your postmaster is. They have to make you believe two rate increases a year is actually going somewhere.
Have you noticed the priority boxes have logos inside AND out now?