I recall Walkup rate is going to be more than the online rate for 1day when i was curious one day and compared. Even day-of. Of course it maybe at the mercy of the vail's profit maximizing computer pricing algorithms if that is always true.
Often some guests are foreigners without internet data access or won't trust in anything but cash, and Vail is going to do their best to suck those tourist dry.
Even if you're there and already parked, once you are sure you're going to go skiing, it's worth it to plug it into your smartphone in the parking lot before you walk over.
If they presented it as they're going to charge you $7 or whatever to buy from a window, you'd say that's ridiculous (even if you're *just* going from 165->172, $7 is $7).
Online rate is guarenteed to be lowest. Ticket window is not guarenteed to be same as online rate (and they will take advantage of that loophole).
At a minimum you'll save time to go to the online pickup versus the line.