Package "Undeliverable" (Sp?} when I've had no issues before?

Okay, I ordered from a company I've been ordering from for over a year and I'm very friendly with the staff. They know me and know where I live. They sent me a package (I bought and paid full price for it) and it told me the package was undeliverable. I live in Crystal City, Missouri and my mail goes to a PO BOX. The mail store closes at 11am. Is there an explanation why this item: 9114 9011 5981 8938 4249 31 was listed as undeliverable? Should I expect it Monday or should I expect it to be returned to the sender and expect it to be sent whenever they get around to fixing the address?

They know me, so there is NO EXCUSE for them sending this package to Eureka. The package is just incense, for those who care or think I'm ordering stuff I'm not supposed to. It's Nag Champa and Moonlight Jasmine incense from the Fred Soll company, in case anyone is interested.

Update : Oh yeah, they HAVE delivered to my PO BOX before and they still do, they said.