Have the game start making an ongoing count of characters sold in rosters.
Each 10 1* characters sold, get a free 2* token.
Each 10 2* characters sold, get a free 3* token.
Each 10 3* characters sold, get a free legendary token.
That’s not a bad idea, sometimes selling a 3* for 500 ISO is a tad insulting, in the vault 3* ISO is worth far more than that.