Running your own business really is at least 90% about customer service. You could have the most amazing widget in the world, but if you don't handle your business properly people won't care. Another thing that is a somewhat icky side affect of running a business is that occasionally you are going to have unhappy customers.
Now, let me turn that around a bit and tell you all that I love my 'unhappy' customers!
I know that sounds weird, but I've been in some form of customer service since I started working at 17, and I can tell you that a mistake or mishap is a HUGE opportunity to make a life-long and very vocally positive customer!
Of course I hate for anyone to not be happy with their purchase, but if it takes a reasonable loss on my part to make it right then that is what I do. And if they choose to leave nasty feedback anyway then that's a reflection on their character, not my service. It almost NEVER happens if you do what you are supposed to ethically.
And if you fix the problem then not only do they like your artwork, but they TRUST you as a person and business owner and you can't get that from a perfect transaction!
Just a little business 411 for ya! :) Have a fantabulous weekend.