Hello, Jay.

I appreciate your time in trying to make the classes work for the Shipping field. I would like to chime in and clarify the details about it. 

The Shipping field functions a little bit differently from the line items under the Product/Service column. When assigning classes, it is separate from the line items' classes.

This is true when you select One to each row in transaction in your Classes settings. With this setting, we're unable to assign the shipping class on the field itself. 

However, when you select One to entire transaction, the Class behavior changes. QuickBooks applies the specified class to all fields, including the Shipping field. We can only use a single class for this though. 

So, in order for us to assign a separate class to the shipping charges, we would need to use a service line item and add it under the Product/Service column. 

My colleague, ZackE, had already mentioned the steps. However, I would like to clarify those steps in detail for you.

Since we've already created a shipping account, let's create a service for it: 

1. Go to the Gear icon, then select Products and services.
2. Click the New button.
3. Select Service as the type.
4. In the Name box, enter something like "Shipping."
5. Under Income account, select the shipping income account you've created. 
6. Hit Save and close.

Now, we have two choices here. 

We can pull up those existing invoices and edit them. Add the shipping service we created on the line item, then assign a class.

Afterward, we would want to delete the Shipping field amount and leave it blank (to avoid redundancy and double-booking). 

Another way is to delete and recreate the invoices. Before deleting them, take note of the invoice details. 

To delete an invoice, simply open one, then click the More button. Select Delete. 

Now, let's recreate the invoice, then add the shipping service item. We can assign the shipping class afterward. 

The Profit and Loss by Class report should now reflect the assigned classes. We can also see that the shipping income is under the correct class.

After correcting the classes, do you also need to send statements for your customer's overdue balances? I'll add this guide if you need help in the future: Create and send customer statements in QuickBooks Online. 

If there's anything else you'd like me to clarify about the Class feature, let me know and I'll help you out. I'll also cover other areas and functions, such as reports, entries, and taxes.