I have possible plantar fascias. pain on foot, worst on arch. i already have flat feet. what is best treatment? what about shoes and pads?

See foot specialist: if your feet are truly flat, there is unlikely to be a best treatment. Treatment should be chosen based on your goals. An active person may not have enough control from shoes, pads, orthotics. However, a properly made orthotic can do wonders. There are also surgical options that a foot and ankle specialist can review with you.