Great post, Thanks for the detail. There is a lot of good info there. To sum up a few points that jumped out at me,

"THINGS THAT ARE MOST LIKELY TO CONTRIBUTE TO JAMS ARE: staples, paper clips, "uneven thickness" (thicker on one side of envelope than the other), "shifting contents "(like a chip in cardboard, flip or a couple keys or slotcards)that shift from side to side in an envelope, contents that are too thick to be machinable (not sure what the standard is anymore... 1/4" to 1/2" I suppose), TAPE, labels and even postage stamps that have lifted and have either created an uneven surface - or (HERE'S TROUBLE) have stuck to another piece of mail, and envelopes in poor condition."

Your above paragraph would indicate,

Keep your envelope CLEAN. If you have to use staples, tape accross them so they can't snag or be grabbed. Make sure that all tape, lables etc. are evenly and completely adhered to the envelope. Rub well across so that there are no loose areas that can peel up. Double check to make sure everything is stuck securely and FLAT If you use glue, keep it off the outside of the envelope.

Keep your contents even. If as Pete(?) mentioned about using cardboard, make sure that it is stiff enough and is the full size of the envelope so it can't slide back and forth, changing the balance or weight distribution, and folds over both sides to keep a consistant thickness or adds for a gradual change rather than an abrupt change.

It sounds in your post like there could be an issue of weight distribution when an object shifts. Especially tokens, heavier chips, coins, keys etc. Disperse the weight evenly accross your piece of cardboard or try and place the weight in the center of the envelope. Secure the contents so they won't come loose, and all end up in one end of the envelope. Reading your discription I can envision a token sliding to one side of the envelope and causing the envelope to shift to an odd angle resulting in the jamming of the machine.

People should remember also, as you point out, damage to an envelope may not even be the shippers fault. Like you mentioned, one other piece of mail could cause a jam and if yours is one of the other 800 envelopes behind it yours could get shredded also. Or, your envelope could be fine but could stick to a loose label on some other letter just before or after yours.

Thus, lableing like you describe, HAND STAMP or putting into a more suitable envelope like bubble mailer will avoid the system that may be the cause of much of this difficulty as the envelope will be hand stamped(?) or processed.

Finally, the suggestion of the glue stick rather than tape etc. is good. I never thought of that. Of course, again, be careful to keep YOUR envelope clean and keep the glue inside the flap and not on the outside of the envelope. You don't want other mail sticking to yours and jamming a machine.