I can imagine the hurdle you've been through to resolve your problem with sending invoices due to a discrepancy with their details, @Craig_B. I'm committed to providing you with the best assistance to ensure a prompt resolution.

If you have followed the suggested troubleshooting steps above to clear your browser's cache and use other supported browsers, I recommend contacting our Technical Support Team if you haven't already. They have the proper tools to look into your account and perform a screen-sharing session to investigate the issue with QTY converting the estimate to an invoice in QBO. 

Follow the steps below to get in touch with them:

1. Log in to your QuickBooks Online account.
2. Click the ? Help button at the top right corner.
3. Go to the Search tab, then select Contact Us.
4. Enter a brief discussion about your concern, then click Continue.
5. Choose a way to connect with us.
   
   

Please check out our support hours to ensure we can address your concerns promptly.

Additionally, you'll want to check out this article to learn how to personalize the appearance of your sales forms in QBO by adding specific information to it: Customize invoices, estimates, and sales receipts in QuickBooks Online.

I'm here to assist with managing your invoice transactions in QBO. Don't hesitate to reach out if you need further help. I'll be available to support you. Bye for now and stay safe!