Sure I know you didn't mention weathershield, and I'm not ctiticising anything you said - I only posted it because I think a long guarantee on exterior paintwork is a bit ridiculous. They have recently changed it to a "lifetime guarantee" and the small print says that the guarantee is for as long as you live in the house. I'm sure that there will be so many terms and conditions around it that it will be literally worthless :- https://www.dulux.co.uk/en/articles/weathershield-lifetime-guarantee
My post was related to a job I did 18 years ago prior to all the VOC requirements, my point was the product is only as good as how it is applied and the condition of the substrate being painted, in the real world guarantees are a safe offer as the manufactures will always find a get out clause. ........... In your reference the Weathershield is the retail version not the Dulux Trade.
You just can't be sure till it's paid. Equally the customer is considering how well you do the job, tidiness, reliability etc. I recently turned down a job, where the customer offered more payment for a variety of additional features. Which to me seemed unneccessary and a bit crackpot. Potentially more costly than he reckoned. Gut instinct said this is unusual. It would have turned the job into more aggro than needed. With him hovering about. It's not good when the customer decides to manage your expertise, if they have no experience. He made no attempt to readdress the situation, knowing he had been rumbled.