This would only be for a specific item. The items have a parent/child hierarchy -- so the first item always remains the same. (sorry if I'm not being clear).
Party Package #1 = qty of 10 people * $10
add on option #1 = (same qty of 10) * $2
add on option #2 = (same qty of 10) * $3
Total cost $150
The reason for this is that I don't want users to be able to change the qty for the add-ons. Each add on must be for the full party.