From the Super Mario Wiki
Level-boost tickets are items in Mario Kart Tour used to increase the skill level of a specific driver, kart, or glider by filling up the gauge to level up with 1. If the driver, kart, or glider has already the maximum skill level, no further level-boost tickets can be used. Silver level-boost tickets can be obtained from Tour Gifts. Golden and amethyst-golden level-boost tickets can be obtained from Tour Gifts as well, but only if a player is subscripted to the Gold Pass. Level-boost tickets can also appear in the shop, but only after the player has gotten at least one driver, kart, or glider to level 6.
"Raise a Normal/Super/High-End driver's/kart's/glider's skill level."
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