I appreciate the amount of detail you've included in your post, devrom. We have a separate support staff that can provide answers to your questions about API within QuickBooks. I'll route you to them for further assistance.

Follow these steps to proceed:

1. Visit the Intuit Developer page.

2. Choose General Questions.

3. Select the Ask a Question button.

4. Sign in with your Intuit log in credentials.

Alternatively, you may find it helpful to explore this article for comprehensive guidance on managing and applying billable expenses to customer invoices directly in QuickBooks Online: Enter billable expenses in QuickBooks Online.

Please let us know if you have any concerns with API limitations within QBO. I'll be pleased to help you with anything you need. Have a great day!