CCW in PO
Here's a good link to check out:
http://www.thegunzone.com/rkba/rtc-usps.html
Your state may have more restrictive laws, in which case all bets are off. However, if it doesn't then it's a gray area.
If you ask the PO, they say no. No guns allowed at all anywhere on the property. Yet they ship guns, so I'm not sure how you're supposed to ship them without bringing them on the property...
I asked the WA attorney general, he sided with the PO but was very wishy-washy about it. I think he was just playing it safe.
I say you can, but my advice is worth what you pay for it

Nobody has ever pressed the issue or become a test case yet and we desperately need one. If you read the law (see link above), there's a part that says "Subsection (a) shall not apply to - the lawful carrying of firearms or other dangerous weapons in a Federal facility incident to hunting or other lawful purposes.". Since licensed CCW *is* a lawful purpose for having a firearm it's legal.
IMO the intent of the law is to simply add a federal charge on to any gun violence actions perpetrated in a PO (or other "Federal Facility"), but the PO reads it as banning guns. It's very poorly worded. Come on, "hunting or other lawful purposes." Hunting in a PO? "Duck stamps! Fire!"