Customer Facing Marketing (Update).
Two alternative approaches to dealing with the persistant complaining customer (i.e. me as detailed here.
One company accepted that I was right and they were wrong (or more specifically "somebody at the service centre doesn't understand how to do percentages") and a refund is coming my way. I conceivably may buy from them again.
The other accepted that I was right, tells me that I should have been kept better informed and then offers me a 10% discount on future purchases within the next six months. I have no incentive to buy from them again.
Right the wrong quickly and you may still have a customer. Act as if you expect repeat business from a dissatisfied customer and you won't.