How to pay commissions to an ex-employee

Severance, is a payroll option. So I'd pay that person the se way you did before they left, I.e. if commissions were W-2, them continue to pay W-2.

If the commissions are earned during her employment and become payable during the severance periods as part of the normal course of processing the commission, it technically is not part of the severance. It depends what was stipulated in the agreement. From an ex-employee relations perspective, you may want to make all payments as w-2 as Wayne suggested.

The transition from W-2 to 1099 is a touchy one, and (imho) the courts haven't decided. See below for a descriptive link.
I believe that Wayne is in essence correct. If her termination agreement indicates that she can continue to "earn-out" commissions for work done during her employment, I would absolutely w-2 and withhold. It costs, but I'm pretty certain that it is the legal requirement.
Ex-employees transferring to 1099 status should be a relatively clean break, and the "severance package" makes this relatively gray. For example, acceleration of stock options upon severance is safer to w-2, even though technically they were not earned during employment. Payment for signing a "release of claims" after termination has been completed is a separate event, and I'd be more comfortable calling that non-income (I still always w-2 it, even weeks later, but it is more clearly on the non-employment side).

http://ghdcpa.com/images/uploads/https---checkpoint.riag.com-app-view-toolItem-usid225ef61b7b98.pdf