If a store provides a customer with poor service, is it moral for that customer
How is this supposed to work? Suppose I go to my local grocery store and I get bad service. Would it be okay to slap the cashier? How about breaking his jaw? Or if we stick with stealing, how much? A bag of chips? A wedge of Stilton? (Could easily be $10.) A bag of chanterelle mushrooms? ($40 a pound would be a very good deal.) What's the rule that decides? Who's the authority if there's a question about what the retribution should be?
There's a reason why we don't let people do what you suggest. There's a reason why it would be perfectly fine for the store to have you arrested if you tried it. Letting people take the law into their own hands makes for chaos. Letting people do things that are against the law for "offenses" that aren't even illegal would be even worse.
You second sentence seems to be meant as an argument for your suggestion. It's true: businesses without customers won't last long. But that means businesses who give bad service are punishing themselves. You also say that if the customers aren't satisfied, the business has no right to exist. What sort of right do you have in mind? Bad service doesn't make a business illegal. And the idea is that somehow the business has no moral right to exist, neither follows form what you say nor justifies what you propose. The business may not deserve customers, but that's not the same thing as saying it's morally wrong for it to keep running and it certainly isn't the same thing as saying it's okay to steal from it.