A: One important thing in negotiations like this is that you don't want to let your emotions take over, which will probably lead to you burning a bridge. Even if you were very disappointed in the initial offer, try to stay composed in your communications and just let them know that this is quite a bit below the offer that you were expecting.
They will probably say that the offer is more than fair when taking into the market, your qualifications, etc., so it is important that you are prepared with a counter offer and some decent arguments to back up your counter offer. Maybe they mentioned in the hiring process or the job posting that benefits would be included? Maybe you have qualifications that put you above the average market employee in this position?
Regardless, employers almost always have some room to move from an initial offer that they make to a prospective employee, so it rarely hurts to ask for a little more.