If a contract was signed as to when payments would be due (upon receipt) and the payee is months past due and in breach of contract for failing to pay and has verbally and in writing suggested he will not pay for services after 100% completion..... can I now present over due and unpaid invoices with a written expectation of chargeing interest on all unpaid invoices if they are not paid within the next 7 days. I ask because There was no agreement in our initial Contract that interest could/would be charged. But the signed agreement does say "due upon receipt" as well as the dates some payments were due in full.