Hello there, Richelle. I can share what I know about the features and options you can use in QuickBooks Online (QBO). 

You have the ability to link estimates to existing invoices if you're using the new layout in QBO. I'll show you how:

1. Open the Invoice and click Manage. 
2. Click the dropdown under Suggestions.
3. Select Suggested transactions. 
4. Click on Add to link the estimates. 
5. Once done, click Save. 
    

However, if you're using the old layout, you have the option to switch to the new experience. 

I can see how the functionality would be beneficial to your business on the old invoice layout. I'd recommend sending a feature request directly to our Product Development team. This helps us improve your experience and the features of the program. 

Here's how:

1. Go to the Gear menu and then select Feedback.
2. Type in your feature request in the description box.
3. Once done, click on Next.
   

You can also track feature requests through the Customer Feedback for QuickBooks Online website.

If you need to make modifications or want to personalize your invoices and other sales forms, follow the instructions in this article: Customize invoices, estimates, and sales receipts in QuickBooks Online.

We appreciate your patience and understanding as we work to resolve any issue you may have. Please let us know if you have additional questions on how to link estimates to existing invoices. I'll be there to save your day.