Congrats on making your first post here in the Community. No worries. I can provide you with some helpful information to answer your question about being able to enter an alphanumeric credit card zip code for payment.
In many instances, the international care holder is given information by the card issuing entity on how to process in the United States when they're presented with this situation. Your customer should contact their card issuing bank to determine what options they have to use their international card with U.S. based businesses.
Our payment processing doesn't use an alphanumeric zip code field. Another form of payment will be needed, if the credit card can't be used.
Here's an article that can give you more details about processing customer payments in your QuickBooks Online (QBO) account: Credit card processing. Check out our tutorials page within the Community to see how to use other features in the system.
I hope my reply finds you with a smile. Let me know if this information helped. I want to ensure that all of your concerns are addressed. I'm only a post away if you need me. Have a great day!