I just was wondering why a 13 oz book with online postage can be mailed from home but one with postage stamps can't be. I've done the online postage and sometimes tried the stamp method because I wasn't thinking at the time. What makes the difference? What postal policy is involved?

Because the online postage can be tracked to you, should something be amiss with the package.  Stamps cannot be tracked back to the sender.

Of course, this doesn't address the terrorists with the online postage accounts...

It's a terrorism policy. Big packages are more likely to contain bombs than small items.

Postage except for stamps can be traced back to the person who bought it. (In theory). Unless you take your package to the counter wearing a disguise and pay cash in the winter, so it's not strange that you would be wearing gloves. But whatever.

That's their policy.

Even if you take it to the counter wearing a disguise and pay cash the post office is covered of any wrong doing because all postal employees taking a package have to ask those magic questions "anything liquard, fragile, parishable, or hazardous."  Tracking you down does not matter as long as they are covered.

ZB

...all postal employees taking a package have to ask those magic questions "anything liquard, fragile, parishable, or hazardous."

And that is just one of many great and reliable policies. We all know how honest terrorists and crazy people are. 

This policy started when the Unibomber was sending packages.   When bombs were bigger.  Way back before we could buy on-line postage.  The on-line postage was excluded as it could be tracked back to the source. 

As already mentioned, it's a Homeland Security regulation that applies to packages untrackable postage .  Just about everything but stamps is trackable these days.

ETA - regarding the 'mailed from home' part of the OP, some rural route carriers will pick up all packages and some will not.  When the carrier refuses, it's usually on the grounds that they don't really know who placed the package in your mailbox.

Thanks. I did get one book back with a sticker on it that sorta explained issues with 'heightened security'. I wanted better detail than that- thanks for filling me in.

I wanted to stick the sticker on a letter to a friend, but just to NOT cause myself trouble, I stuck it inside the letter, just in case.

Thanks again!