I’ve always put my cards “right side up” with a piece of paper then tape on the toploader and have never had an issue in the hundreds of transactions I’ve done over the years. That being said, if I had a card shop in the area, or even had supplies I would be more inclined to go team bag route, put for now this does the trick!
To the OP: Anybody who uses tape inside a mailer is either clueless or a COMPLETE DIMWIT. They are the same people who use phone booths and betamax’s. They are too ignorant to think of a better alternative method.
Like Benoit said, a cardsaver will do the trick. Or cover in cardboard and wrap with sandwich wrap.
Tape has destroyed more collectibles like cards and comics than the paper drives during WW2.