My Advice:
Be honest - point out the TRUE problem. If it's your fault put it right. If they are still not happy - walk away!!!
HONESTY I feel is over 50% of our our qualities - it will pay off in the end!
Most reasonable people will see the benefits of WFP and it will weed out your moaners who, lets face it, are not worth the hassle.
I have been WFP for just about a year (end of this month). I am now oversubscribed again!

So it doesn't bother me if someone tries to be clever with me and treat me like s***. I just walk away, politely. Never give them a mouthful, you only drop to their standards

David