You may think you are getting a really good deal on that new vehicle after negotiating with the salesperson. Don't forget the next step though, you still have to deal with the auto financer. If you are not prepared, you could end up paying alot more than you had expected.
It is the auto finance managers job to get you to pay the most possible so that the dealership can maximize their profit. You need to either get a pre-approved loan elsewhere or know your credit score and what rates you should qualify for.
They may also only want to offer you packages with add-ons that you don't want or need.
You really don't need the extended warranties in most cases. As it is, the bumper to bumper usually lasts 36,000 miles or 3 years. The transmission warranty usually lasts up to 75,000 miles. If these are not enough for you, haggle with the financer, their warranty mark ups are usually 100%, therefore they have room to come down on the warranty costs.
Another add-on they may offer is paint protection. All this really is is a "grand" wax job. Something you can do yourself at below a fraction of the cost.
Just be smart and know your choices. Do not let them talk you into anything.