Don’t do it! 9/10 the dealer won’t know sh*t about detailing. The paintwork won’t be properly decontaminated and cleaned prior to applying their coating. And the product may just get slapped on with inconsistencies etc.
The last car I bought I said no to the full Autoglym coating package but they then went and did it anyway for free in the end. I was so pissed off because it was done horribly, swirl marks, the lot. I ended up having to do a full decontamination, not only to clean the car properly but to remove the shoddily applied wax coating!
I’d suggest researching some stuff about detailing, buy the products you need and do it yourself or if you don’t have the time or inclination but do want the car to be protected make sure you get a proper detailer to do it for you.