Quintas: IMHO the folks are clients until the contract is fulfilled. Buying an extra opener would be a customer. Also, Scarlett67’s estimation of the situation is one that I agree with - even though she does not have a contract with every client. (The company I worked for would have to have had a contract with every client, however - one of the upsides to working freelance I guess 
)
I’ve sold between $10 and a couple thousand dollars worth of stuff to accounts* that have been purchasing from the company I work for sometimes 10 years or more (sometimes only a couple of weeks, or even days though)…they’re all customers to me (except when I want to sound important).
*“Accounts” because I didn’t want to use “customer” or “client” at that point, and hell, they’ve all got account numbers. 
What if you sell mobile homes?
Never paid much attention to it before, but it’s true that we call our large, long-term accounts “clients” and the rest do tend to be “customers”.
Thankfully our “clients” don’t seem to think we’re assholes, or we likely wouldn’t be calling them anything.
“Have you noticed that people who refer to their customers as ‘clients’ tend to be assholes?”
Some people just happen to be assholes.
Some have clients, some have customers.
Um, yeah! My whole industry refers to our clients as… well…CLIENTS!
Not very many people (that I know of) in our industry (environmental consultants) are assholes.
I’m going to make a wild stab in the dark here and guess that what you meant to “get at” was that people who are pompous, fakey and self-important are assholes and that using the word client, when it’s not really applicable is just one ASPECT of that assholishness???