We were notified by a supplier that the contractor we are doing work for has a record of not paying customers. Our contract with them has a waiver of lien rights, and we just started the work this week. Can I send a letter of commencement? And what can I do to ensure we are paid and can file a lien if needed. Do we have any way to ensure we are paid? We've also been notified that if we submit an invoice more than once, we will be charged. What can we do?