$400 seems like an awful lot. I occasionally get a return here or there on most of the sites, but if it was such a large amount of money that would be alarming!
I've had a sprinkling of returns (bought a different size typically) over 5+ years, but the only place I
ever got a chargeback for credit card fraud was FT. It was never $400 thank heavens.
I complained about it at the time (and at the beginning they didn't even have any accounting for it in the stats; the money just went away) in part because it was many months old. How on earth can it take them such a long time to uncover these problems. And why is it that they're the only site that makes these chargebacks?