you cant really send just a portion of the cart price to paypal, the entire cart price needs to be passed.
if your existing items already have a price, then you need to create discount rules to discount those items so that only the cake and the deposit is passed over.
you could create a new cart column for itemType to make the discount rules easier to create.
For items that should not be passed to paypal set the itemType to 0
For items that should be passed to paypal set the itemType to 1
then create the discount as:
number of items with a specific column value:
if the number of items where 'itemType' has a value of '0' is '>' a value of '0'
Based on items with a specific value:
total of 'TotalPrice' where itemType' has a value of '0' 'plus' '0'