So I went to book the trip I was looking at on Travelocity, and when I got to seat selection, I wanted to pick one of the E+ exit row seats. However, the site won't let me pick that saying that it "must be reserved at airport". Does choosing that option really mean I have to wait till checkin to book the seat (and risk losing the seat that I want) or (since I'm a MP member and I'd be inputting my MP# with the reservation) will I be able to immediately go into the reservation through United's website and pick the seat I want?