Should I negotiate my severance? - 6 months and counting

The company sent a severance agreement with a release of claims, and I am unsure what to do. I do not want to leave money on the table. Did anyone negotiate and succeed?

Ask for the offer in writing and do not sign on the spot. A day or two to think is normal.

I asked for two more weeks and a neutral reference. They agreed to the reference and added a week.