My take is slightly different. First, I would ask what the dealership protection actually involves. It may be that Mazda are well aware of how the rust will develop on the Mk3.5 and know that a less forensic treatment is perfectly adequate these days since their manufacturing processes have gradually improved on this model. They will presumably have liaised with the dealers on what is involved in the additional treatment and figured out the right pricing, just as they would on say, official Mazda rubber mats or alloy wheel upgrades. If you are keeping the car just for the first three years I would say the dealer protection will likely be sufficient and there will be a value to having the main agent invoice for the benefit of a semi-casual buyer (which let’s face it, is most MX5 buyers) looking for a three year old MX5 who is aware of the rust issues but not completely obsessed with them. Then they can add their own protection at a later date if they wish (they probably won’t bother). If you are keeping it for longer, then perhaps an underbody assessment after three years and take it from there. But the initial dealer protection sounds like a good, official and not too expensive start to a long-term protection regime.