Yea, when I was planning my build Ryan from ebrewsupply warned me about that. Unfortunately, the key switch is only rated for 5 or 10 amps (something like that) and therefore requires a contactor that can handle the juice required by your set-up. It would be the same scenario no matter if you used a key switch or a regular 2-way switch.
p.s. If that's the only thing you forgot to order you are in VERY good shape! Every body likes to pay extra shipping and handling charges anyway.