I have had this happen to me several times, and I finally had someone at my local post office explain it to me.
I don't really remember the details, but "received" and "accepted" are actually technical terms for the Post Office, and they mean different things.
This probably isn't accurate, but it's something along the lines of: the former means "it's been dropped off" and the latter means that they have actually processed it in some way.
So I think your package is sitting around at the post office.
