Basically, and most people pretty much follow this, anything bigger than seeds should be shipped in a padded envelope at a minimum. Seeds can be shipped in a standard envelope, with padding added in the case of bigger seeds.
Having said that, there are ways to bend that around. For example, I just received D. scorpioides gemmae, and they were in a regular envelope. Scorp gemmae are some of the bigger gemmae, so they could have been in a padded envelope, or even a small box. However, the one who sent them to me packed them in such a way that even though they took three weeks to get to me, and they were machined at least three times (despite having a non-machinable stamp and having "non-machinable" written very clearly on it), they arrived in a still moist paper towel with only minimal losses. Most of them were actually sprouting.
Cuttings should be the same, padded envelope at a minimum, small box preferred.
Now, having said that, I have, no fooling, for real, for really real, actually received a whole Cape, an entire plant, albeit a juvenile, in a standard envelope. This one took almost six weeks to get to me (I don't know the story there), and the plant along with the cuttings that accompanied it were completely flat but still green. Definitely packed with care. I don't even want to guess how many times it was machined. However, being Capes, the plant and every cutting grew multiple strikes within about a month.
Maybe I got lucky.
In my experience with sending and receiving plants, seeds, and all their accouterments, a good rule of thumb to follow is:
- Seeds, small (Drosera, Ping, Nepenthes, some Byblis, etc.): regular envelope, with a cardboard frame if wanted
- Seeds, large (VFT, Sarracenia, Darlingtonia, some Byblis, etc.): minimin regular envelope with extra padding and cardboard frame, a padded envelope is better, maybe a box for large amounts
- Gemmae: minimum in a damp paper towel in a sealed plastic bag in a cardboard frame in a standard envelope, padded envelope or box would be better
- Cuttings/pullings: Drosera - minimum in a wet paper towel in a sealed plastic bag in a cardboard frame in a standard envelope. A padded envelope or small box is better. larger or thicker Drosera, Pings, VFT - minimum in a wet paper towel in a sealed bag in a padded envelope, a small box is better
- Plants, all, it doesn't matter how small, potted or bare root: in a sealed plastic bag in box at a minimum, big enough to contain the plant without too much constriction. You'll need to arrange packing material as needed to secure the plant inside the box while minimizing damage potential from handling en route.
For all the above, except seeds, write "Live Plants" somewhere if possible. There are some employees of the various carriers that will take extra care if they see it. Even though I have an opinion of the USPS as a whole, I've also seen my postal carrier with boxes that I've sent, or I'm receiving, up on the dashboard to get some heat in colder weather.
Definitely not the end-all for shipping plants, just my thoughts based on my experiences. Maybe look at it this way; ship it to them the way you would want them to ship it to you. Plant-based "Golden Rule".
Everything happens for a reason. Sometimes the reason is that I make bad decisions.