Did you modify the Total Price Calculation from:
[Price] * [Quantity]
to:
[Price] * [Quantity] + [DinnerPrice]
for this?
you should also create another calculation:
realPrice:
[Price] + [DinnerPrice]
then edit the code that sends the item price to paypal to use the realPrice instead of Price.
are you using paypal payments pro? or paypal standard?