Thank you for reaching back out to us, Shalo. I appreciate that you made sure your customers are set up for online payment and answering the questions my colleague asked to assist you better. If your customers are receiving an error message, here is what they can do to correct this.
Over time, the collection of data from frequent page visits can cause corruption of data. Clearing cache and cookies will remove those corruptions, allowing the page to load a clean copy. Depending on their browser, please encourage them to follow this link to clear those cache.
Second, have them right-click the Review and pay button in the invoice email and copy the URL. Open a web browser and paste the link into the address bar. If the link still doesn't open, suggest to them to try another web browser. QuickBooks Payments work well with Chrome, Firefox, Internet Explorer, and Safari.
Third, have your customers ensure their web browser is up to date.