They stay in your sideboard until you grab them with a learn effect, or another kind of card that could look for them outside the game
If sending just one or two cards, consider taping the card-holding top-loader(s) between two more empty top-loaders, for extra rigidity and peace of mind (ensure tape is applied at the sides, so there is no way for the card(s) to come in contact with the tape)
What set is Skrelv, Defector Mite from
TCGplayer's published standards allow some creases at Moderately Played or lower depending on severity
Japanese Release Date : December 6, 2019 Key Cards & Chase Cards SR Professors Research, SR Zacian V, RR Stonjourner VMAX, HR Morpeko VMAX, UR Quick Ball Gold Condition Brand New and Sealed : Factory-sealed booster box guaranteeing authenticity and completeness