The thumbnail version:
- You will inevitably have unhappy customers from time to time.
- How you deal with them will directly affect future business.
- But you don’t want to retain them at all costs.
The full version:

The unreasonable customer. Let them go.
They will always turn up, the unhappy customer. I’m not a believer in the old adage about the customer always being right because, not only is it untrue, but if that’s the prevailing belief, then it invites unreasonable behaviour from those occasional customers from Hell.
But assuming your shop has a reasonable customer with a reasonable product complaint, then whomever has to deal with the unhappy customer could do worse than adopt Starbucks’s LATTE approach (Listen, Acknowledge the problem, Take action, Thank them, Explain what you’ve done).
Any reasonable person should respond well to LATTE. If they don’t, they may not be reasonable, and you may not want to retain them. Life is too short to spend time battling with energy-sapping, unreasonable customers.