I don't really trade much on here, but I do enjoy sending PIFs, and most of them are of the PWE variety. For one card, I'll drop it in a toploader with a piece of painter's tape over the top (pull tab included), wrap a small piece of paper with a note on it around it, and mail it with one stamp.
For 3-9 cards, I use the 9-page sheet cut into thirds, and stick up to three cards per pocket. I take two pieces cardboard cut up from work folders and form a pocket around the "page". I stick a note in the PWE, put a regular stamp and an "additional ounce" $0.15 cent stamp on it, and off she goes - into the mailbox outside my office, not at the post office. I apply the stamps myself, at my desk.
For anything over 10 cards, I use a team bag, sandwiched between two toploaders (ideally old, nasty ones) and use a bubble mailer. Here's the important part, reiterated: the only time I go to the post office is to buy stamps and to mail bubble mailers. PWEs are mailed by me, from my hand into the mailbox. Since different postal clerks decipher shipping rules different ways, it's a crapshoot when you go in with a PWE (in my opinion). There have been stories on here of members going to the post office, getting told a PWE was $3.00, and going to another one across town to be charged $0.55.
As far as I know, no one has had any issues with my PWEs; if they have, they unfortunately haven't told me anything. I use the same drop box outside of my office building each time, because apparently my shipping method, from that location, doesn't raise any of the "letter vs parcel" questions. Like I said, I've had no complaints from any recipients, though it's possible that they've just been too polite to raise the issue when it's a PIF. I've also never had one returned for insufficient postage (yet - knock on wood).
Also, round about way of answering your initial question - in my experience, it is exactly as Jim (jimetal) said in his response. "No PWE" means either the trader doesn't want to deal with just trading a few cards at a time, or they don't want to deal with the risk of getting the PWE chewed up by post office machines. At the end of the day, it comes down to communicating with your trade partner how you plan on shipping and how you prefer to receive the cards. Even if they don't specify one way or another, it's always helpful to tell someone that you plan to ship PWE so that they know.
I'll never quit collecting entirely, but I am downsizing. Check out my COMC store and help me thin out what I don't want so I can buy cards that I do want. See something you like? Send me a message on here, and we can knock the price down quite a bit. I'll even take a bit of a loss if it means getting you a card you really want.