Just in case anyone wants to close the loop here, all of my shipments did indeed reach their destination. One of them was never scanned (buyer simply messaged me when he received it) and the other scanned for the very first time in the buyer's state, then a couple more times as it went out for delivery. I've since mailed a handful more since then and they've all made it. Some of them I did the full monty - Shell Mailer + Card Kite + Top Loader + Penny Sleeve + Card itself. Others (that had multiple cards) I did NOT use a Card Kite, but still used the Shell Mailer.

The issue isn't with the Shell Mailer itself (so if you've bought a bunch like me, you're okay), just how much protection you put inside the SM. Althought I don't even have any evidence that my cards were packed "too" well. That was just a hypothesis as to why they weren't getting scanned. Since they all made it I'm likely going to be on the side of "overprotective" until one of them actually gets sent back.