Nothing in this act shall be construed to prohibit a public utility from filing a schedule or entering into any reasonable arrangements with its customers, or prospective customers, which provide for a minimum charge for services to be rendered, or from providing for any other financial device that may be lawful if the schedule or arrangement, before becoming effective, is filed with and approved by the commission. The schedule or arrangement shall be subject to revision or modification on the part of the commission upon complaint or its own motion.