Shipping Cell Phones

So, I was at my local post office yesterday to mail off my rent check. While there, a lady was at the window next to me asking the mail associate about shipping cell phones. They told her that as long as the battery was in the cell phone and not separate, then she could mail it.



When I asked them "So, cell phones and batteries can be shipped, but ONLY if the battery is inside the cell phone?" and they said yes.

Now I know PartTimeScrapper could answer this for sure, but how you supposed to ship buyers cell phones and the batteries separate in the box, if the USPS wont allow this to happen?