When a Mii reaches Level 20, there will be some changes in what happens when they level up after that.
After this level is reached and a gift has been given, the Mii will say, "Thanks for being so nice! Please accept this token of my gratitude." They will then hand you a coin. It can be silver or gold. You cannot do anything with these coins except sell them at the Pawn Shop. The silver coin is worth $100.00. The gold coin is worth $200.00.