Hi there, info-3hgroup-gma.

The option to enable a pop-up that prompts when you enter an item on an invoice is unavailable. However, when you click Save on an invoice you just created and hover your mouse to the QTY column, the current Qty on hand will show. You can refer to the screenshot below for your reference. 

I'll surely take note of this suggestion. Meantime, you can also run a report where you can keep track of your inventory. Let me show you how:

1. Go to the Reports menu.
2. In the search field, enter and select Product/Service List.
3. You can click the Customize button to add more columns.

I'll be attaching these articles as well as your guide in managing your inventory:

• Set up and track your inventory in QuickBooks Online
• Add inventory products in QuickBooks Online

Please don't hesitate to mention me in your reply if you need more help. You take care always and have a great day!