So not knowing where you are from, but my experience here in a suburb of Philadelphia...
I have had a dozen or so trades with a member of this site from western PA. Almost all our trades are PWE. usually 2-4 cards per trade. I would say 75% of our trades go through. the others never arrived.
Recently I have had two trades to other states get mauled in delivery. I pack my cards in sheets and will send no more than 27 per trade max and usually try to limit to 18 for safety. I use cereal boxes to "protect". I have talked to the post office and will typically take their word regarding postage and feel that 99% of the time he puts a regular "forever" stamp on them and throws it in the bin. NO extra postage, NO hand cancelation.
Wouldn't you expect the expert to know if it should be hand canceled or extra postage? After talking to other traders here, I have since purchased a few sheets of "2 oz extra stamps" and a stamp stating "non-machinable". Since doing this I have had much better success with getting my mail through.
The post office does not care about you or your mail. They are a business first and foremost. Like all businesses who have lost focus on the customer...