I'm shocked that they offered to cover any of it. And as you're finding out, Toyota's are prone to rusting. But really anything is here in the Northeast.

When I bought my 2013 Trail, that front shield was missing. I drove around like that for a good year and a half before I replaced it with an aftermarket skid plate, and it was fine. The only reason why I replaced it was that I intended on doing some off-roading. If you don't plan on doing any off-roading, you really don't need it.

I'd also recommend, if you plan on keeping this 4Runner long-term, look into getting the undercarriage sprayed with some sort of a rust preventative like fluid film or wool wax. Not any of the black gooey rubberized things, they just make things worse.