Hello there, @KevvyB, 

I'll provide the details regarding your concern. 

QuickBooks Online (QBO) uses the FIFO concept, which considers the first items you purchase as the first items you sell. The Cost of Sale is also likely to be zero since you process the invoice before receiving the inventory from your supplier. Here's a sample screenshot below: 

I'd suggest tracking your profits per product by creating an invoice after you've received the inventory from your supplier. Since your business has the other way around, it'd be best to consult your accountant. They can provide the appropriate workflow to ensure your inventory reports and books are accurate. 

Also, I recommend running the Sales by Product/Service Detail report to view all your sales transactions for your stock item (see the screenshot below). Just go to the Reports menu and enter its name in the Search box. For more information, visit this article: Check Your Sales And Inventory Status in QBO. 

Lastly, you can click the Customise button on the report to focus on the details that matter the most to you and your business. 

I'll be right here to help if you need further assistance. Keep safe always, and stay healthy. 

Best Regards, 

Raymond Jay