From the Super Mario Wiki, the Mario encyclopedia
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 reached 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 subscribed to the Gold Pass. Level-boost tickets of a certain class can also appear in the shop, but only after the player has gotten at least one driver, kart, or glider of that class to level 6.
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