Severance pay is not last-paycheck pay. Paycheck pay is for work already performed. Severance is (in the US) generally down to agreement between an employer and employee (and the employer normally dictates the terms because the employee in most cases lacks a bargaining position) or as a way to comply with laws mandating notice periods for layoffs.
Beyond returning your laptop, severance pay has a few more strings attached, like waiving your right to sue. But, everything is negotiable.