Should I negotiate my severance? - 3 months and counting

My manager said the package is final, but the letter gives me time to review. There are 18 people counting on me, so I am trying to be careful. I do not want to leave money on the table.

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.