I used to ship micro-sd cards in plastic cases through the mail, here in the UK. At first I used standard DL (letter) envelopes, since they are cheap and the case inside the envelope didn't push it over any depth size limits.

However, when I got two angry support emails for having sent them "empty envelopes", I had to ask them for photos, which both showed a small sd-card-case sized hole along one short edge, with some tell-tale marks. What was happening was the leading edge of the envelope was going through some kind of thinly-spaced rollers, pushing the case to the rear end of the envelope, and then the rollers had such a grip on the envelope that they squeezed the sd card out through the corner of the envelope like a squeezing a pip out of a lemon.

So I had to move to padded envelopes, which were then a more consistent depth over the whole length of the envelope, and so they worked fine in the mail machines. But that upgrade ate into my margins since I was only working on a small scale.

It's little details like these cause vague statements like "It is normally recommended to use bubble wrap to protect SD cards in transit" - lessons learned the hard way!