You don't have to be onsite to buy -- you can do it all from home, rather than taking time out from your trip. Altho, I'm sure you know that, and have some reason for doing it in person while you're there. I'm sure you'd still get the current incentives (except maybe the ice cream!). (But, if you're concerned about the timeframe -- given that any kind of transport can break-down, or even at WDW accidents can happen to get in the way, you might want to allow more time -- if everything goes smoothly, I expect 2 1/2 hours is enough, but why have to stress about it?).