The math justifying this is extraordinarily simple, and I'm shocked at how uncommon FSA continuation is especially if you are planning to use the FSA balance during a short period of time. I expect the underutilization of extended FSA benefits under COBRA has a lot more to do with people not understanding how the rules work for it. I personally experienced the following:
I left my job in January of a plan year to take a break for a few months before starting another role May 1. I had enrolled in a $2000 FSA plan and made one payroll payment of $167 as I knew I would likely be needing some pricey dental work in the early part of the year. I continued my benefits, including FSA, through COBRA for the months February, March, and April. Including the admin 2% I paid an additional post tax contribution of $170/month during which I accumulated more $2000 of expenses which I was reimbursed for fully. I paid a total of $677 of mixed pre and post tax contributions and received $2000 in reimbursements during my 3 months of COBRA. If anyone doubts the math works for extending FSA during COBRA with post tax contributions they're not trying very hard and are probably providing very poor advice on the subject.
