They are liberal letting you buy certificates. They Only looked at one of my oems to confirm model. Didn't ask age. In some states,cost is variable based on cost of a tire. CO AZ. In others,seems to be a flat rate. CA.
I would recommend for 20 and 21". Those are 95% of the issues here.
I bought the Discount Tire certificates for Air with 20" tires for about $200. No real tire failures but I picked up a nail in the left rear and had a slow leak. It was fixed for free. Then I picked up a screw in the right front in the corner of the tread. It could not be repaired, and they replaced the tire for free. Be aware that the certificate is only good for one tire replacement and then has to be "renewed" for that tire for continued coverage. Cost about $70 for the renewal. Cheap insurance for a $450 tire!