The safest solution is not to make the snap decision to purchase anything while physically in the finance office. The dealership has control of the discussion in that environment and it’s not conducive to good decision making on the buyer’s part. As a general rule, if you haven’t looked into something beforehand, then don’t buy it based on the finance person’s spiel alone. Practically all of the items they offer in the finance office can be purchased later from the dealership or otherwise after you’ve had time to look into pricing and value.
The only reason to really buy anything in the finance office is if it’s something that you know you want and you would like the convenience of having the cost rolled into your monthly payment. If this is the case, then do your homework beforehand to get an idea of going prices for those items before making your deal. That knowledge will give you back control over the discussion and you’ll be in a position to negotiate for a fair price.
On a related note, also watch out for ‘dealer add-ons’ to the vehicle as those are also big profit items for them (they’re generally of questionable value and/or are marked up to ridiculous levels). These are generally minor things outside of the standard manufacturer sticker that the dealer ‘adds’ to the vehicle before offering it for sale (usual culprits are pinstripes, nitrogen-filled tires, or vehicle locator devices). The dealer will ‘assume’ those into the price offered because they’re already physically on or in the car and many buyers will just accept it as fact.
You’d be surprised as how easy it is to make those costs go away by telling the dealer that you don’t want them and it’s their choice to either 1.) physically put the car back to how it was delivered from the factory, 2.) find you the same car without those add-ons, or 3.) not charge you for them. Considering that it would cost the dealer more money to remove the pinstripes or to refill the tires with normal air, etc. and that the dealer has a clear preference to sell the cars already in their inventory, I haven’t found a dealer yet that would chose something other than option 3.