Here's my take on this, for what it's worth. I have carried an amex platinum for more years than I'd like to think about. Moreso than other card companies, I view them as a partner, or somewhat like a spouse, if you like. So, just as in anything, if I am doing things that worry my partner, I try to alleviate the issue and soothe the concern. In your case, I cannot see what is to be gained by standing your ground. I think the reply I would have given, and in fact have done in a similar situation, is to pay the bill immediately and build the trust that makes for a good partnership.
With Amex charge cards, I have found it best always to be able to pay, regardless of whether you owed 100 or 100K. I keep this in mind always.
I am not faulting you as you have every right to do what you did. But the underlying message you conveyed to Amex was that you did not have the money now, but you'll have it in two days.
The NPSL is the upside, but it's a partner thing, earned over time.
