You can let your customer to open the email through a private window and pay the invoices from there. This helps us verify if the problem is with the browser he/she is using. To use a private browser, here’s how:
If your customer can pay the invoices using a private browser, you can let them clear their browser's cache to start fresh. For the detailed steps, you can check it here: How do I clear my browser cache and temporary Internet files?.
To learn more about letting your customers pay invoices online, you can check this article: Let your customers pay invoices online.
You can always drop a comment below if you have any other questions. I'm glad to help!