In the add to cart button, you will give an ID for the cake.
On the charges tab, you can created a charge to check if the cake ID is in the cart, if it is, then add the $50 deposit.
Say the cakes ID where 22, you would create a new calculation:
triggers:
Presence of an item in the Cart
If item '22' exists in the cart
calculation:
flat rate charge of 50.