I'm here to help you make sure your customer would be able to send payment, evanferree1.
To help ensure that your client would be able to pay the invoice via their AMEX card, you'll need to set up the AMEX credit card to accept payment.
For more detailed information about this process, you can read this article: Add or remove AMEX credit card processing for a QuickBooks Payments account.
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Please let me know how things go, evanferree1. I am more than happy to help you.