A knee-lift bar (sometimes referred to as a Free Hand System-FHS) is a hands-free mechanism on some sewing machines that moves the presser foot up and down by moving your knee, it is not an item we sell.
The question regarding if you use a knee-lifter bar refers to the placement of your sewing machine on the platform (how far to the left/right and front/back does the machine need to sit on the platform). There are 1000s of sewing machines and they each have a different placement for the knee-lift bar, if you use this mechanism it is possible your custom insert will need special placement. The additional fee is a guarantee that the insert we create will not interfere with your use of the knee-lift bar on your sewing machine. Forgoing this charge and we cannot guarantee you can use your knee-lift bar on your sewing machine in the cabinet with the insert installed.