How do I enter an invoice so that it appears under a particular income account on the profit and loss - ie revenue versus fee for service?

Thanks for getting back to us, Michelle2018. 

Allow me to jump into this thread and add some information to the awesome answer provided by @john-pero. 

As what @john-pero mentioned above, the income account is linked to the products/services you sell. The invoice doesn't have a specific field where you can select a particular income account. 

Once you set up a product or service items, QuickBooks Desktop will ask you to select an income account (see screenshot below). This will be used to track and handle each transaction, so they'll translate to your Profit and Loss report. 

All you need to do is create an invoice, then select the item you want to use. The program will automatically post it to the income account you've initially designated to the item. For a visual reference, I've created a workflow of how QuickBooks handles those in the second and third screenshots below. 

To create an inventory or service item:

1. Go to Lists at the top menu bar, choose Item List.
2. Right-click the page and select New.
3. Enter necessary details, then pick an Income account in the Account drop-down.
4. Click Ok.

Should you want to review how the accounts work in the background, open the Transaction journal within the invoice:

1. Open the invoice you've created. 
2. Select Reports at the top of the page. 
3. Click Transaction Journal (see screenshot 4).

Here's a link to our QuickBooks blog about understanding items in QuickBooks:

Understanding Items in QuickBooks

These steps and resources can give you an overview of how the program manages your transactions. 

Please know that you can always count on me if you need help with anything else. I'll be around, anytime.