Thanks for visiting the Community today, @Zbates. As an insight, the Item details section is used to itemize the bill such as to enter a description of the product or services. 

On the other hand, the tax on your purchases is already paid and filed by your vendor. Hence, there's no need to add it to your expense transactions. 

You can simply enter the items you purchased and enter the total amount you paid. 

Should you need to track the tax portion, you can add it to your transactions as another item. I'd also suggest you check this with a tax advisor or accountant to make sure your tax reports are correct. 

For future reference, you can browse related topics about expenses and vendors for QuickBooks Online.

Don't hesitate to reply to this post by adding some details below if you need anything else with Sales Tax. I'm here to help.