I've got some instructions to share on how your customer can pay their invoices successfully by clicking the Pay Now button.
There are times that you can experience unusual behavior in QuickBooks Online or anything that you access through the browser. Having an unresponsive function of the Pay Now button can be caused by the stored cache on the browser that your customer is currently using.
To get this sorted out, I suggest having your customer open the invoice link using a private window. Doing this will let him access the link in a clean slate.
If it's working fine, the customer can go back to his regular browser and clear its cache. This way, the gathered data will be eliminated to prevent this issue from happening in the future. You can refer to this article for complete details about this process: How do I clear my browser cache and temporary Internet files?.
You can also visit these links below for more information about sending and paying invoices in QuickBooks Online: