A strategy to consider if your vehicle is still under warranty and you are going to buy the extension - don’t buy it now but wait like the previous poster suggested.
Put the cost of the platinum plan in an equity like a quality stock, dividend paying stock (ET is oil/gas stock that pays 8% dividend), or S&P index fund and just wait. When the factory warranty is ready to expire liquidate the investment and buy the policy. Statistically you will come out positive and most times it will be a couple hundred bucks on a $4k investment if it goes more than 8-24 months.
Of course market could crash but
There would have to be a serious market crash to be down a couple hundred bucks on that $4k investment.
Or, just invest the cash as you suggest and
skip the service contract, and pay your repairs out of that $4k investment...my guess is at the end of ownership, if you do all the regular maintenance, you'll still have principal remaining, as well as the some of the gains.
(The above must be true on the whole, or nobody would sell a vehicle service contract...they do it because it is a financial product, and they laugh all the way to the bank. Statistically, they take in and earn more on the money they collect than they pay out, and do this even after employing administrative staff, CEO's, buying office space, etc)