Cake Display? where is that?
You start with the display stand that holds one cake. Then you can upgrade to a stand that holds two cakes, then three cakes. Upgrading beyond that will get you a refrigerated case that's "built in" underneath counter. The first such upgrade will hold two cakes, and you can then upgrade to one that will hold three cakes.
The advantage is that cakes in there carry over to the next level, so you can kind of get a jump on things!
When the last person leaves the shop, don't pick up their money right away. Instead load up the case with the most expensive cakes you can make. Then pick up the money.
When the next level starts, you'll already have those cakes made.
Hope this helps!