Let's go through some troubleshooting steps to address the issue your customers are experiencing with the Manage Payment link, kc103.
Before we get started, please confirm if your customers encountered a specific error message when accessing the link. Any additional details you can share will help us identify the cause and deliver the best possible resolution.
One possible reason your customers have trouble using the Manage Payment link could be that their browser has accumulated too many cached files. QuickBooks relies on the browser's cache to function efficiently, so clearing any outdated files can help fix the aforementioned matter and improve the overall system performance.
Ask them to click the links below for the detailed steps of different browsers:
If this issue continues, I recommend clicking the browser's Refresh icon (please see the screenshot below) or using other supported browsers.

You may also want to generate sales reports in QuickBooks to determine your best-selling products, gain insights into your buying and selling activities, and analyze your cost of goods.
If you have other concerns about managing your sales and income transactions, don't hesitate to drop a comment below.