Back in Colorado we used to have to separate metered from stamped mail if we mailed it at the post office. In fact, there was a slot for Priority Mail flats, one for metered letters and one for stamped letters. I say Colorado--I should specify the Vail area. I noticed on mailing something at a Denver post office it just said all mail and they didn't seem to care if you separated it.
Well, after I moved to Tucson I note that our local post offices all say "All Mail" and don't seem to care if it's separated out . What I can't figure out is why the change? Why don't they care anymore?
There once was a time when they wanted it put in two boxes--one for Local Mail and one for Out of Town. Those vanished a long time ago.