Hi, @info-thekerryman. Thank you for your quick response, and I appreciate you for using a new browser to narrow down the result. I'm here to provide additional troubleshooting steps to fix the issue.

If you haven't yet, I'd suggest signing in to QuickBooks through a private window to check browser-related issues while handling your credits. Here are the keyboard shortcuts to save you time:

• Google Chrome: press Ctrl + Shift + N 
• Mozilla Firefox: press Ctrl + Shift + P 
• Safari: press Command + Shift + N

Once done, try to pay your invoice and see how the credits behave. If it works fine, I'd suggest clearing the cache of your main browser to remove browser history. 

If the same thing happens, contact QBO Support for further assistance. This way, a live representative can look into your account securely and investigate the behavior you're getting.

For more tips about customer credits, check out this article: Create and apply credit memos or delayed credits in QuickBooks Online. On the other hand, if you need to refund an amount to your customer, see this link as your guide in tracking the entry: Record a customer refund in QuickBooks Online.

Let me know if you have any other questions about your customer credits. I'm always here to help. Have a good one!