Actually, it has to be done at Guest Relations, and yes, it will be $35.
On the technical side, if you add on options, they actually have to process the transaction as an upgrade. (They void your ticket, credit you, issue you the new and charge the difference between the two)
Guest Relations are found at each of the 4 major theme parks. There are also locations at Downtown Disney (1 at the Marketplace, and 1 at the West Side), as well as 1 location at the International Gateway.
Have a fantastic trip!