On these rare occasions, I always treat them in the way I was taught many years ago at my first ever seminar. The speaker said...
"The Customer's Perception is the Reality".
It didn't make sense at first, until he clarified. But this is what it means ...
"If the customer 'BELIEVES' he got a bum deal from you, then he DID get a bum deal from you".
And over the years this has happened a few times. That's life and can't do anything about that. But each time, I remember the words above as I go thru my routine and come out the other side smiling, with a new life-long raving fan customer.
It's no good getting on your high horse and arguing the point with this customer, because that can only ever end badly - for you.
If he believes he got poor value for money, then that's the story he will pass to his buddies. And you know what they say, - bad news travels 10X faster than good news- right? This is why it's essential you put this right - properly and immediately!
So when this happens, I encourage you to view it as an opportunity to reveal to him what's possible. Pull out all the stops and ask this person "what he would like you to do to put it right for him?"
Most times, he'll simply appreciate your offer and not ask for much at all. On other occasions, you might get asked for a refund - in which case you hand it over immediately and with good grace and a 'thank you' for trying. But even better is to get them on skype, call them and OFFER things over and above what they've paid for. Offer a bonus product for FREE complete with an extra 6 months support or whatever. Completely blow him away with your generosity.
This will be totally unexpected, because most people don't behave like this. By nature we tend to think that 'we' are always right. And we justify our stance by telling ourselves that... "No-one else has complained about this product so it MUST be OK - so why are you complaining?"
Don't fall into this trap of believing you're never wrong. Being 'right' is a subjective thing, and your 'right' could well be another person's 'totally wrong'.
Don't treat this as an inconvenience. Treat this customer better than you would do the non-complainers.
If you do this as I've described, I promise you pretty much any unhappy customer would come away from this conversation feeling like a million dollars - on an absolute high. And provided you do the things you've promised to do, together with follow up emails to make sure he is now happy, then chances are they will now be a raving fan forever.
Even better, they'll be happy to tell their friends about this unbelievable level of service they received from YOU.
Try it. It works