The site you're thinking of is
The Gun Zone, and the website is
http://www.thegunzone.com/rkba/rtc-usps.html. According to this article, concealed carry is perfectly okay inside a post office if it's not in violation of state law.
Another point of view posted with
Buckeye Firearms Association, website here
http://www.buckeyefirearms.org/content-96.html states that 18 USC 930 does not apply to US Postal Service facilities, but rather 39 CFR 232.1, which makes it a violation.
Interestingly, the signs posted at post offices cite 18 USC 930.
A firearm may be shipped through the mail under certain circumstances, but you can't bring a firearm into a post office, but in order to ship it you have to bring it into a post office. Typical government Catch 22.
Anyone care to invest a small fortune on getting the issue resolved in court?