Summary
American Express is one of the strictest issuers on spending behavior. It is sensitive to sudden spikes and same-day velocity, especially on new accounts, and large single charges near the limit are likely to be declined without prior notice. Amex prefers gradual ramping, steady usage, and a heads-up before unusually high purchases.
Best practices
Keep utilization under about 40% of your limit.
Start with smaller test purchases, then scale.
Notify Amex in advance of a large transaction.