Mo. Rev. Stat. § 407.915: Civil action for all claims against principal may be joined — express or contract waivers of commission laws, invalid.

1. Nothing in sections 407.911 to 407.915 shall invalidate or restrict any other or additional right or remedy available to a sales representative from seeking to recover in one action on all claims against a principal.

2. A provision in any contract between a sales representative and a principal purporting to waive any provision of sections 407.911 to 407.915, whether by expressed waiver or by a contract subject to the laws of another state, shall be void.