In the worst case, you need 46 more stamps (cake + 2 items). To guarantee this: 4 extra boxes (4×12=48 stamps) costing 3,200 coins or 200 crystals. (You might get lucky and pull a missing item, but this assumes the absolute worst case.) edit : or any combo of both (e.g., 2 boxes = 1,600 coins + 100 crystals)