Hello, My gf and I (we are both in our mid-20's) are going to Orlando for four days. This is our first time. My gf is a huge Harry Potter fan and I am excited about many of the rides at US and IOA. We want to do both parks on Sunday 12/16 and use the Express Unlimited option to not wait on any lines. Unfortunately pricing on the US website is about $400 for the two of us!! Suppose I were to buy 2 discounted both-parks tickets on eBay or from a discounter? Would we be able to go to the gate on Sunday morning and pay the retail-rate to add on the Express Unlimited passes? Or to get Express Unlimited do we have to buy the entire package (park entrance + EU) at the gate? Thank you!!