Even after i update the customer contact email address, when i send them a new invoice goes to the old email even though its been already removed from the system. why??

I'm here to assist you and work towards resolving this customer contact email issue you encountered.

I have tried replicating your scenario and successfully updated a customer's email address, and then when I create new invoices, the updated email will be applied automatically.

As it appears that the scenario I tested does not match your specific case, let's take a moment to double-check the customer's email address and ensure you save it. This ensures that the updated information is available for future invoices.

Additionally, you can update your customer's email address using a private browser or incognito window to see if it will auto-apply to your future invoices.

Moreover, you can check this article that provides comprehensive instructions on including different components in your sales form and personalize them according to your specific preferences.: Customize invoices, estimates, and sales receipts in QuickBooks Online. 

You can always reply in the comments below if you need further assistance managing invoices. I'll be right here, willing to help at any time.