How do I add stock without it messing up my expenses when I have already purchased the products?

Hi there, @craig-ad.

You can enter an stock quantity adjustment without recording a purchase or sale so it'll not mess up your expenses. Let me walk you through the steps.

1. In your QuickBooks Online, click the + New button and select Stock Qty Adjustment.
2. Enter the adjusment date from the field.
3. Select the appropriate account in the Stock adjustment account drop-down.
4. Sleect the products in the Product field drop-down.
5. For each item, enter either a new quantity or a change in quantity.
6. Enter the details about the adjustment from the Memo field.
7. Click Save and close.

Here's an article you can read for more details: Adjust stock quantity on hand in QuickBooks Online.

You might also want to check out this article to know the impacts of stock tracking on balance sheet and profit & loss reports: Impacts of stock tracking on balance sheet and profit & loss reports.

Please keep in touch if there's anything else I can do to help you succeed with QuickBooks. I've got your back. Have a great day!