The Universal Express Pass can be helpful, but it is not recommended to buy it in advance. It is date-specific and non-refundable, if you change your vacation, get there and crowds or low, it rains, etc. you do not get your money back. The first two weeks of December it will probably not be that crowded. If you get there and decide the lines are too long, you can go into many different stores in the park and buy one.
The Express Pass allows you to skip the regular line once at each participating attraction. It is not a "front of the line" pass, it is a special line for onsite guests and those who have purchased the passes.
There are only a few attractions in IOA that do not accept Express: Harry Potter and the Forbidden Journey, the Ollivander's wand shop show, and Pteradon Flyers. Hippogriff and Dragon Challenge, the other two WWoHP rides, do accept Express.
There aren't a lot of discounts available for one-day tickets, but Undercover Tourist sometimes has tickets a little cheaper than other places. You can also get discounts if you are a
AAA member, and the Universal website's tickets are cheaper than buying them at the gate. Some people have also been able to use credit card reward points to get free tickets, if you have a credit card with rewards, you might check to see if this is an option and how many points you need.
I think there is a sticky on here that discusses transportation, that you might want to check out.