Let me be the bug in the ointment. If you haven't had your car washed in a year then don't bother with the clay and wax THIS time. Get it washed, wait a day or two, get it washed again THEN have them clay and wax/seal the car. You will not remove a year's worth of crud with just one washing and they won't be able to properly clay bar the car. Any wax they apply will trap the grit/oxidation which they couldn't remove further into the paint.

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“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now