I personally have no problem with PWE's, especially for most of the (25+) trades that I have made. Most have been "small" trades involving 6 or less cards. I have also used top loaders, which I am reading is considered a no-no. When doing that I have placed cardboard, not corrugated cardboard- think cereal box, on both sides of the envelope. If there has been an issue with my packaging or shipping, no one has told me about it.
I think an important aspect that hasn't been mentioned is keeping the envelope or bubble mailer a consistent flat shape. My Post Office has delivered 10+ envelopes to me in the last year and a half that weren't flat enough to go through their sorter. It then gets charged as a package and I am asked to pay for extra postage. I actually received one today, for the first time, in a small, 4x6? bubble mailer. I didn't have exact change so I'll have to pick it up tomorrow. The package should contain 7 cards and from the shape, they appear to be in a team bag in the middle of the bubble mailer. It might just be my Post Office, but they want the envelopes to be flat, period. I've complained, but I think we all know what the Post Office does with complaints.
I also always take my envelopes or bubble mailers to the Post Office for them to weigh and determine the postage.