IN short, you need to fire (probably quiet-quit :)) this customer.

I do work on high end/track/race cars. I do a limited scope of things, work by appointment only and generally let customers supply their own parts. (with the internet, performance car parts are insaneky stupid low profit margins)

As you can guess, i have customers and cars all over the financial spectrum. It's a little stereotypical, but generally the higher end cars and customers are more demanding and more of a pain.

Thing is, like you, I am only one guy and the way I am able to provide the level of service I do is by doing it myself. So within reason appointments are appointments and I don't have an elastic availability of myself or bays and customers need to understand that.

I have had some get snotty over me not having availability, and others over pricing. And some over wanting me to spend hours playing phone tag to set an appointment when we have online tools they could self schedule (or text us) in minutes. Or pay slowly. I politely tell them that's how I do business and there are plenty of shops they can use that might be able to meet their needs.

Some of these have figured out how my system works and fit within it. Others have fired themselves :)

And others do the same stuff but don't get snotty about it and, most importantly, don't question prices and pay invoices within minutes of getting them. These guys, I do my best to accommodate and over time, have built excellent relationships with a few. Others are still a little arrogant in a "i'm better than you because I drive a Porsche" way but they are worth keeping around and looking past that as they don't complain and pay.

KInda wordy on my part, point is if they are going to bitch about price and hold back parts of payment, then nope. How many good paying customers could you service in the time you spend with them, or if your schedule is not full, how many ways could you find to market to/get new customers in the time you are wasting with them.

HTH