Looking at more robust Sales invoicing and Accounts Receivable tracking functionality. Does anyone have a favorite app they recommend to integrate with QBO?

Welcome to the Intuit Community, qbo. 

While I’m unable to recommend you an app, I can route you to the website where you can look for an application specific for your business needs. From there, you can integrate the app to QuickBooks Online (QBO).

Here’s how to find an app externally from the QuickBooks App Center:

1. Go to https://apps.intuit.com/.
2. On the Search box, enter Sales Invoicing or Accounts Receivable (A/R) tracking.
3. You can also select any categories from the left menu.
4. Then, choose the right app that suits to your business.

You can also find an app from within your QBO account. Here’s how:

1. Login to https://qbo.intuit.com.
2. From the left menu, select Apps.
3. On the Search box, enter Sales Invoicing or Accounts Receivable (A/R) tracking.
4. You can also select an app by clicking the Browse category drop-down.
5. Choose the right app for your business.

I’ve added a screenshot below to show you the third and fourth steps. 

Once you found the correct app, it can extend account functionality and provide additional services for your business.For more information, you can check out this article: Find Apps.  

If I can be of further assistance, please add a comment below. I’m glad to help, qbo. Wishing you continued success and to your business with QBO!