Severance offer, what should I ask for?

My manager said the package is final, but the letter gives me time to review. I have been trying to stay organised about it. 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.

Thanks for writing this out. I went through something close to it 10 months ago, and it got easier once I kept a simple log of every call and form.