I'll add a note about postage in response to u/usdballum's comment. The WaxPackHero post is good, but they tell you to use not enough postage on a single card, and too much on multiple.
Buy Non-Machineable stamps at the Post Office (the ones with the Butterflies in that Imgur link) and also get some Additional Ounce stamps. They're $.24 each and are better than wasting Forever Stamps on ounce #2. That shit adds up.
For BMWT, you can use PirateShip.com to print labels so no need to wait in line, AND it's cheaper! If I'm selling on eBay, I just ship through them because it's as cheap and a bit easier to get the label printed.
Don't wait in line to drop off PWEs. The Clerks may give you shit for the envelope being rigid. Just drop it in the lobby slot.
I use these PWEs for 1-2 cards. Even with the comic book board I use to protect the cards, it stays under 1oz.
I put one or two cards into a single Card Saver and sandwich with the comic book board cut down to just squeeze into the envelope. No painter's tape needed (except over the Card Saver opening. I put the card in upside down so if it slips out of the penny sleeve it doesn't get stuck on the painter's tape)
https://www.amazon.com/gp/product/B09WMF7V68
I use these PWEs for three or more cards. I put the Card Savers side by side and use painter's tape to secure them to the comic book board. Even one card in one of these is over an ounce, so you'll need that Additional Ounce stamp.
https://www.amazon.com/gp/product/B07D2BMDW7
I use this cardboard for BMWTs. Yes you can just cut up other boxes for free, but I always have some at the ready, and one sheet cut in half is the perfect size. I also cut the corners so it slips in and out a lot easier.
https://www.amazon.com/gp/product/B0B58VGZLN
I use these padded mailers for smaller stacks of cards. Packed like u/usdballum shows
https://www.amazon.com/gp/product/B0B96RB6BF
And I use these for larger stacks. You can cut them up so that a flap of the envelope can fold over and get tucked inside so that both sides have the same amount of padding versus just folding the envelope over. Yes it's overkill, but I'm a little insane.
https://www.amazon.com/gp/product/B09MZ4RKBD
Some people may think my way is a bit much, and yes you can get away with less most of the time, but I can't recall an issue with any package I've sent.