I can see how the functionality to link existing invoices to sales orders would be beneficial to your business, EdNewman. I'm here to share a few details about this.

We take customer suggestions as opportunities to improve the various features within our products. Therefore, I'd encourage sending a product recommendation directly to our Product Development team. This helps us improve your experience with the program. 

Here's how:

1. Go to the Help menu.
2. Choose  Send Feedback Online.
3. Click on Product Suggestion.
4. Click on the drop-down for Type of Feedback and select Product Suggestion.
5. Select the Product Area.
6. Then enter your thoughts and suggestions.
7. Once done, select Send Feedback. 

For your visual reference, I've attached some screenshots below.

Additionally, I'm adding this article to further guide you in managing your business growth and income transactions in QuickBooks: Help Articles for Sales and Customers. It includes topics about sales transactions, managing customers and payments, and customizing forms, to name a few.

I'm all ears if you have other concerns about managing your sales transactions, EdNewman. Please don't hesitate to reply here or create a new thread. I'll be right here to assist you.