This. The Tire Rack site says the shaving service is $25 - $35. A tire shop may be able to handle it. This makes for a strong argument for self-insuring.TireRack actually has an option for this. You can buy a tire, tell them the tread of your existing tires, and they will shave the new tire down to the depth of your current tires before shipping to you.
Just some things to consider regarding this. I don't have AWD but I think I'll skip the certificates in the future since I've thought of this, and just get a shaved tire from TireRack or a new tire and pay out of pocket if I get non-repairable tire damage in the future. Well, I don't have AWD so I could just stick the new tire on the rear and not rotate my tires for the rest of the life of the tires too.
If the tires are already down to 3/32 it wouldn't pay--time for a new set anyway. If the tire spec is 11/32 and they're worn to 10/32 you wouldn't bother with shaving a replacement. Somewhere in the middle makes buying one tire and having it shaved a good value proposition. It would be worth it to investigate what the manufacturer says about acceptable differences in tread depth. Some AWD examples are at the bottom of this link:
This is not just an AWD issue. FWD vehicles also should not have significant differences on the front tires--traction control, stability control and anti-lock braking can be thrown off.