By no measn let on you know what was there. It may be a setup. (BTW - I also believe there was a second shooter on the grassy knoll in Dallas, who was a trained sniper. )

Complete your contract. When it comes up for renewal or you don't have one, include your payment penalities in the contract and as part of every invoice. If they don't agree, respectfully decline to renew and move on. You might included disputes go to binding arbitration.

With these kind of clients, the third letter would be from my lawyer explaining they have 24 hours to courier payment plus penalities or the contract is being terminated. Then file a complaint with the BBB.

Not much you can do unless you have very deep pockets for the lawyers.