Thank you for posting here in the Community. I want to make sure your customer will able to pay invoices without any problem.
Are your customers using Internet Explorer? If so, there's an open investigation about unable to pay invoices. The case number is, INV-38337 and our Product Team are actively striving with them to get this settled.
Let your customers perform the workaround given by our engineers, which is accessing their accounts using Firefox or Google Chrome. Once they are logged in, re-open the invoice and click the pay now option to reprocess the payment. For the detailed steps, please browse this article: How to let customers pay their invoices online.
If they can successfully pay using a different browser, they'll need to delete temporary internet files on their default browser. This process aims to speed up your overall browsing experience.
However, if the workaround didn't resolve the issue, I suggest contacting our QuickBooks support so they can update the ticket. And attach your company to the record of the affected users. We will send an update once the issue is resolved via email. We are open Monday-Friday 6:00 AM - 4:00 PM and Saturday 6:00 AM - 1:00 PM Pacific Time.