The ep is a separate ticket and not attached to the park ticket
Decide now if you want to change your one day p2p ticket into a bundle
If you bought your ticket from UO, it is possible to do so
The ep prices can increase if you wait to buy it the day you are in the parks
If the park is crowded, price can be higher than it is today
If it is pouring rain the day you are at the darkside, you won't need the ep
Not everyone will do the parks in bad weather