>>57370776The vast majority go in sleeves and then in some sort of top loader. Some people sandwich them between two pieces of cardboard or get one of those "Shipping Shield" sleeves, but that last one is rare. Then those go in envelopes and that's that. In practice, there's a lot that can go wrong here: automated sorters are heavy machine and an envelope will definitely get put through it. If that goes wrong, it'll get crumbled to shit. Water damage could easily get in, even if you sleeve it, top loader it, and tape some cardboard around them. But I've gotten a ton of singles shipped to me and not a single one was damaged by transit, even off the continental US.
When people are shipping more than one card, especially when they're not particularly expensive, they'll shove them all in a single sleeve/top loader. That feels like a poor idea to me, but I've never noticed any damage from it aside from how tedious it is to extract them. No clue about the other direction, when the card is worth several hundred dollars. Haven't ordered one of those, but presumably they get pretty serious with protection.
