I e-filed with a payment plan that was approved, and provided my bank info. Do I still need to mail the voucher? If yes, do I include a check?

You do not need to mail the voucher if your payment plan was approved and your payment is made by bank withdrawal. If you are making the payments by check, you need to include the voucher. Change the amount due to match the check you are sending.

Thank you! Just saved me a trip to the post office, I was about to get my shoes on.