Do not get undercoating on a new vehicle. First of all, new vehicles don’t rust very much - manufacturers have gotten much better at rust resistance.
Second, it’s grossy overpriced.
Third, it can actually promote rust if done poorly.
Fourth, it makes the vehicle harder to work on and inspect. It can actually lower the value of your vehicle because an old trick when selling a used vehicle is to apply undercoating to hide pre-existing rust or frame damage. Always be wary when buying a used vehicle that has what looks like fresh undercoating, or even any undercoating at all.
The same goes for all the other crap the dealer will try to sell you - fabric protection, extended warranties, VIN etching, yada yada. These are all grossly overpriced and often the dealer will make more from tacking on this garbage than he’ll make from the sale of the vehicle in the first place.
Negotiate your price first, BTW. Once you have it, in writing, THEN decline all the other crap. If you tell the dealer you don’t want any of that stuff up front, he’ll have less of an incentive to give you a decent price for the vehicle. Of course, you don’t want to lie, but don’t volunteer the information. Similarly, if you have a trade-in, don’t work it into the price. The dealer will use this to game you - if he thinks you care more about getting good value for your old vehicle, he’ll offer you an awesome trade-in price - but work it back into the price of the new vehicle. If he thinks you need a great ‘deal’ on the new vehicle, he’ll offer you one - and then give you peanuts for your trade-in.
If the dealer asks if you are financing, or have a trade-in, or are interested in the extended warranty and all that, just say, “I don’t know yet. I want to see what kind of price you’re willing to give me on this truck before I make those decisions.”