As a professional business they dont overschedule for any given class and therefore you may not be able to jump if they are fully booked. With an appointment - you are guaranteed to have a slot available (weather permitting) and minimize your waiting to a 1.5 - 2 hours window from arriving to actually leaving, depending upon which load you are assigned to.
You can just show up, but this won't guarantee you will be able to jump. I'd recommend doing the prepay thing on their website. Especially this time of year.
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