My experience has been similar. Three orders for $1000 each went through; 4th order for $500 in the same 30 days was cancelled without explanation (the only way I found out was to go to their website and click "track orders").
Has anyone who's had a cancelled order been able to later buy more cards from them? Is there some sort of unspoken limit?
This would be a good site to use because of the low prices and personalization of the cards (they look more like credit cards than anyone else's). But the "random" unexplained cancellations are obviously a problem.