We filed electronically for both state (IN) and federal. Indiana has a form (PFC letter) with a coupon. Since we e-filed, do we also need to send in this form?

No, you don't have to send in that form.  That form can be used to pay your tax due.  You can send in the coupon with the check needed to pay the amount due, or you can follow the instructions on the form to pay the IN Department of Revenue online.  The form is an aid and may be used, but you don't have to use it (but you do need to pay your tax amount due, whichever method you choose to do so).