Should I negotiate my severance? - any advice?

I was offered 12 weeks of severance after 12 years as a procurement assistant, and I have a few days to sign. I worked as a procurement assistant before this. I do not want to leave money on the table. Did anyone negotiate and succeed?

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

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