How can I add a non-stock product to our sales item list. Doesn't seem to be a category under New on Products & Services?

Stored temporary internet files could be the reason why you can't see the non-stock category, communityoil.

Let's try accessing your Quickbooks Online account using a private window. There are times that the stored cache can cause performance issues in the product.

Check out these shortcut keys:

•     Press Ctrl + Shift + N for Google Chrome
•     Hit Ctrl + Shift + P for Internet Explorer or Mozilla Firefox
•     Hold Command + Shift + N for Safari

If you're able to select the non-stock category, you'll have to go back to your regular browser and clear the cache to speed up your browsing experience. If the same thing happens, you can use other supported browsers.

For your future reference, you can read this article about tracking services and non-stock items.

If you have any additional questions or further clarification, please don't hesitate to tag my name in the comment section. I'll always have your back.