The problem is this is not a customer, it's a problem waiting to happen.
You already know what you should do, it's just a bad thing when someone puts you in this position.
Tell him you don't work that way, it causes too many problems for you and him. If he accepts it, great, keep working with him. If he protests, or says everyone is doing it, tell him you don't work that way, and move on. You don't owe him an explanation. It's not the type of customer that is good for your business.
Don't worry about him, he'll find plenty of techs to help him out and take his money. good riddance.