I agree with FF, the credit card validation FAX that Xheli is asking for is no threat to your card account - if you cover the first 12 numbers (that they have anyway). My one order with them was about $225 last Spring, and they never did this validation - but it's not a bad idea for an amount like $600. It is nice to know that they are doing this, to protect your card account from being mis-used.
My local BBB will tell you when a company joined and if they are still a current member. Most companies that I've ever researched have a "satisfactory" rating - never run across one that had anything less than perfect, but Xheli probably does do business with a ton of people and may have had problems under earlier management? No horror stories about them recently...
I would think you'd be safe dealing with them for this order, especially at those prices!
Cheers,
Dave