Could someone please walk me through how to refund a customer's American Express payment (through QuickBooks Online)? I've tried several times with no luck.

Good day, gulfcoastpropert.

I'd be glad to walk you through recording a customer's refund in QuickBooks Online. 

Here are the steps:

1. Go to the + New button.
2. Select Refund Receipt.
   
3. Enter the required details in the fields.
4. Click Save and close.
   

I have an article here for your reference: Record a customer refund in QuickBooks Online.

Using the steps above, you're able to record a refund for the customer's credit card payment.

You can always tag me directly if you have any other QuickBooks concerns.