I followed the video, but it was unclear to me if ALL the cards get sandwiched between to pieces of cardboard, or if EACH card needs to be sandwiched between two pieces of cardboard. We will be sending in about 20 cards.
All the cards will be fine.
I've always sandwiched the whole lot of that size between two pieces of cardboard. I've never sandwiched each card individually for any order of multiple cards.