Customer has issue opening invoice to pay. Error: 400 Bad Request (request header or cookie too large)

Thank you for sharing the exact error your customer encountered, @aaron-aites.
 

Error 400 occurs when your browser sends cookies or header data that exceed the limits set by the QuickBooks Online server.
 

To fix this, let your customer open the invoice in a private or incognito window, which prevents data from being stored in the cache. This approach can help identify the problem. You can use these keyboard shortcuts: 
 

• Safari: Command + Shift + N
• Mozilla Firefox: CTRL + Shift + P
• Google Chrome: CTRL + Shift + N
• Microsoft Edge: CTRL + Shift + N

If using a private or incognito window resolves the error, return to your regular browser and clear the cache, as the error may be caused by corrupted cache files. Otherwise, your customer can try a different supported browser to fix the problem as well. 
 

After the customer has completed the steps above and the issue persists, share this article below for additional troubleshooting tips and solutions:
Fix payment errors for customers paying invoices online

If you require further assistance with helping your customer open an invoice to make a payment in QuickBooks Online, please leave a message below.