700+ trades completed, 90% PWE probably and I've had 1-2 reports of damaged cards. I've never recived cards, on a PWE trade, that arrived damaged (and I've seen some pretty beat up enveloes arrive in my mailbox)..
Like others, I can't afford to use a bubble envelope for every trade, my wife would quickly divorce me once she saw how much I was spending on postage.
As others have said, buy a good scale, know the USPS postage rates, buy your own stamps, and you are good to go.
I also avoid postal clerks at all costs.