Severance offer, what should I ask for? after 14 years as a copywriter

My manager said the package is final, but the letter gives me time to review. I would rather hear the honest version than the polite one. I do not want to leave money on the table. Did anyone negotiate and succeed?

Thanks for writing this out. I went through something close to it 13 months ago, and it settled down once I kept a simple log of every call and form.