Is there a way that I can allow my clients to pay via Amex?
Welcome to the QuickBooks Online community family! It sounds like you're using QuickBooks Payments to easily process your customers' payments online. I'll be happy to share more on this.
QuickBooks Online Payments is available for Canadian currency transactions and enables your customers to pay via Visa, Mastercard, Discover cards, and Apple Pay. Here's an article for more details on that: QuickBooks Online Payments overview
Because the Payment feature isn't available on multi-currency invoices, including USD, it wouldn't be able to process US currency credit cards. As a workaround, you can either use a third-party app in the Apps menu of your QuickBooks account or another service provider outside of the program. This won't offer the exact same options of being able to click through the invoice to pay, but will at least make sure you're able to accept payments from your customers regardless of their currency.
I hope this helps! Don't hesitate to reach out if you have more questions.