How do I add classes to old invoices that contain charges for multiple classes?

You have to enable the Class feature and select the One to each row in transaction option for Assign classes, vickisouza.

There are two ways to assign a class in the transaction, either One to entire transaction or One to each row in transaction. To apply classes for each line item within an invoice, you have to select the One to each row in transaction option.

Here's how you can set this up:

1. Go to Settings and select Account and settings.
   
    
2. Navigate to Advanced and choose Categories.
   
    
3. Enable the Track classes option.
   
4. Select One to each row in transaction for Assign classes.
5. Click Save and Done.

After turning on the class feature, you have to create a class. Here's how:

1. Go to Settings and select All lists.
   
    
2. Choose Classes.
   
    
3. Click the New button to add a class.
   
4. Enter the name of the class and click Save.

To assign classes in your existing invoices, follow these steps:

1. Go to Sales and Get Paid.
2. Click Invoices.
   
    
3. Open the invoice by clicking View/Edit.
4. Add a class for each individual item on the invoice.
   
5. Click Save to apply changes.
6. Repeat this process for any additional invoices where you need to assign classes.

Feel free to reach out here if you need any additional help.