How should I ship cards?
A soft plastic sleeve (like the ultrapro or dragonshields used when playing a deck) in the mail is not enough support, and cards shipped that way that I have received have come in damaged. The best way to ship is in the rigid plastic top loaders, after that between two pieces of cardboard or poster board, something more rigid than the card(s) in the middle that will protect what you are shipping. Almost all cards I have received are one of these two ways, and none of those have ever been damaged during shipping.

If you use cardboard or poster board, you need to tape the pieces together, so they do not slide apart and the card slide out during shipping. A single piece of tape over the opening of a top-loader can also help, particularly with some top loaders that are larger than others. There is less risk of the card sliding out of the top-loader than there is of it sliding out from between two free floating pieces of cardboard, but the risk is still there.

As for what you ship them in, any envelope card size or larger will do. Magic the Gathering cards are Poker* sized cards are 3.5" by 2.5" and top-loaders are 4" by 3". The smallest envelope according to USPS rules that qualifies as a letter (for standard stamp usage) is 5" by 3.5" so anything around that size works perfectly to fit a top-loader.



*Yu Gi Oh cards are slightly smaller Bridge size cards at 3.5" by 2.25"