The highest single charge I've made on the card was close to $15,000 for a watch. Being able to buy something like a Rolex or airplane with a charge card is indeed cool but also a bit dangerous if you aren't careful. Especially when you realize they expect you to pay it off with the next billing because it is a charge card and not a credit card.
I wonder sometimes how expensive a purchase has to be before they throw the flag and want to discuss it first. When I had a green card I bought a new computer with it and it was around $2400. I think it was 1989 or 1990 when computers were not cheap. Anyway the clerk called the charge in and then handed the phone to me. I had to talk to the AMEX person before she'd approve the charge. They say 'no pre-set spending limit' but I'm sure there is a dollar amount that I can't cross until they get paid for my current charges.