Hey there, louie hyd. Let me share some steps on how to add a subtotal for your taxable items in QuickBooks Desktop (QBDT)

When adding a subtotal for your items in the Invoice transaction, ensure that you created a subtotal for your taxable and non-taxable items. Right after, you can add the percentage in the line item, and then next to it is the subtotal. 

Here's how: 

Step 1: Create a subtotal for your Taxable and Nontaxable item

1. Go to the Lists menu and click Item List. 
2. Click the Item dropdown, and press New or just hit the Ctrl + N. 
3. In the Type dropdown, choose the Subtotal option. 
4. Enter the Item Name/Number (ex. Subtotals for taxable).
5. Tick the Columns to total. 
6. Once done, click the OK button.
7. Repeat the steps for creating a Subtotal for Nontaxable items. 

Step 2: Add the Subtotal and Tax 

1. In the Customers menu, click Create Invoices or press Ctrl + I. 
2. Fill out the necessary details such as the Customer, Class, Date, Terms, and Due Date. 
3. Select an item by clicking the dropdown in the Item column. Ensure that the taxable and nontaxable are separate. 
4. If you enter Taxable: 
   • Ensure that the next line item is the Tax. Click the dropdown and choose your tax. 
   • After adding the tax, make sure that the Subtotal is next to it. Press the dropdown in the Item column and add the subtotal you've created for Taxable Items. 
5. If you enter Nontaxable: 
   • Ensure that the items were added first before the Subtotals.
   • Click the dropdown in the Items column and add the name of the Subtotal for Nontaxable. 
6. Once done, press Save & New. 

Furthermore, you can also record an invoice payment when a customer pays you for an open invoice. 

If you have further information or concerns, please enter it in the comment section. I'll be happy to assist you.