I think it has something to do with the duration between semi-large purchases. A few years back, I charged $10,000 to a restaurant on Amex (downpayment for wedding) with no second look. In December I bought $200 worth of contact lenses and 15 minutes later tried to buy a pair of Aldens from Shoemart over the phone and Amex blocked the purchase. And they were seconds, goddamit!
4/30/14 at 2:05pm