They stack. I started with a $5, then went up to a $25 one by adding $20, then added another $25 to get my baller sign. Assuming things haven't changed (I doubt they did), just add combinations of signs to your cart (so buying another $5 will give you a level 2 sign, buying a $10 sign will give you a level 3, buying 2 level 2s will give you a level 4, and so on).