I've been told that a 2 oz. stamp will get a letter treated like a non-machinable letter. However, as your post suggests, it varies from one clerk to the next. If I have a PWE that isn't too thick (1/4" maximum for a 'letter') and weighs, for instance, 2.2 ounces. I will use a non-machinable stamp (butterfly - 75 cents each, 55 cents + 20 cent non-machine surcharge), plus an additional 2 x additional ounce stamps (20 cents each). The total for the 2+ ounce 'letter' of which I do NOT want machined is $1.15. If 1.01-2oz., you can get away with one butterfly + one addt'l oz. ($0.95), or if </= 1oz., you can use just the butterfly (or whatever may be pictured on the non-machinable surcharge stamp at the time, right now, my PO has California Dogface butterflies, so when my wife asks me if I need anything from the PO, I tell her a sheet of butterflies). 2 ounce stamps are also 75 cents each, which is where there may lie some confusion between clerks. The post office definition is: A nonmachinable mailpiece is a mailpiece that must be sorted outside of the standard, automated mail process. One example where a stamp of this nature would be needed (i.e. cards in a 9-pocket page), it is too rigid or contains items such as pens, keys or coins that cause the thickness of the mailpiece to be uneven. This doesn't necessarily include any old letter that would require a 2 oz. stamp. This is the reason why I pay the regular postage necessary for the ounces of the PWE + the nonmachinable surcharge, which is, of course included with the first stamp placed on the envelope.
I could be totally wrong but things have been working so far. I may even be putting too much postage ($0.20) on each PWE I send out, but I'd rather put on too much than not enough. However, paying $4.00 for a parcel rate for a 2 oz. letter, I think that clerk is way off base.
Side note: funny story, well 2 related stories actually about my old man. He retired about 15 years ago from the USPS, he was a letter carrier in a small town (~15-20,000 pop.) for about 30 years. I have great respect for the man, mainly 5 different walking routes (rotating) in NWPA, enduring the 4 wonderful seasons we had living just south of Erie, almost winter, winter, still winter, and construction. I used to get the old telephone book out, (remember those?), and pick out a name or an address in the city and he could tell me the address if I gave him the name or he could give me the name if I gave him an address. He was like Raymond Babbitt. Second story, recently I was prepping some PWE to ship while he was visiting, we were shootin' the sh!t, and I handed him the envelope and said, "what'd ya think, is that an ounce or less?" He holds it in his hand and kind of lifts it up and down, guaging it on his mental scale, and says, "no way, that's 1.2 I'd say." I break out the scale and well, I'm sure you can guess what it weighed. With a cool grin, he says, "I thought so."