These days I think it's more like 15-20% of your customers are inconsolable mouth breathing morons. Also these days it's not "OK" to fire customers, as everyone needs to feel like they are important and it is somehow wrong to not let them tie up a HUGELY disproportionate amount of resources even though they will still badmouth whoever they are doing business with. Try one reasonable conversation, if it doesn't work get the product back while refunding them in full including shipping (so you can counter the negative feedback) and move on. Happy customers don't tell anywhere near as many people as unhappy customers but when reasonable people research they will see that your business is ethical and really does try to satisfy everyone (just not to unjustified lengths).