Upvote:2
I have booked a few of these, and unfortunately the package deal means that the flight times are fixed (as rates are seasonal). You may choose to come later but you'll have to pay any differences up-to and including a full fare cost.
If the package is being offered directly by the airline, you have a better chance of changing the flight portion. I did this once - the same package instead of buying it from an agent I bought it directly form the airline and I was able to select two different flight itineraries without any additional penalties - and it ended up being cheaper than the agent's quote.
Bottom line - ask your vacation provider.