Hello susan30,
We can resolve it by letting your customers paying the invoice using a private browser. If they are using Chrome, please click on the Hamburger icon, choose New incognito window, open their email from there. Then, click on the Pay Now button in the invoice.
I'll personally report this to our engineers for review. You can also contact our Support Team so they can further investigate the cause of it. Please click on the Help menu, click on Contact us, enter a topic, and select a way to reach us.
If you have any questions, please let us know. We're here to help.