Not a chance :)
It could seem to some people that such a stance is easy, but no matter the strength of principle, when you see your business go offline and customers start banging at the doors for you to sort it out then the situation becomes more complex.
So from this customer - keep up this stance, keep being transparent and I for one will stick by you guys without hesitation!
DDoS mitigation isn't that expensive antway considering the size of FastMail.