If you're only trading one rather valuable card, I recommend that you just put it in a soft sleeve (penny sleeve) covered by a top loader (hard plastic but thin case) and then place it between two pieces of card board from an old box that you can cut up. It makes it impossible to accidentally bend, and is very secure.
If you're trading a few cards a bubble mailer would be ideal. You'll end up getting a bunch of these anyway from buying and trading so you can reuse them by putting a new address label on and resealing the edge that was opened.
For these options you'll want to take them to the post office so that the proper postage is applied, or if you have an accurate scale you can do it online.
Yeah, you can use an envelope for sending a single, but it's just not very secure. I'd only do that if you're trading a single minor little holo rare or less.
Personally when I sell I either use a bubble mailer or put it between two pieces of recycled card board (you can find old card board all over the place, just look behind businesses that are getting rid of it for recycling, but be careful behind stores/shopping centers as there are often creeps back there >_>, so watch your back and have an adult with you)