Is there a way to add a note to an invoice, just for our reference? So, for example "spoke to customer, they will pay on X date"?

Welcome to the QuickBooks Community, Rmaxwell2 !

I appreciate you connecting with us here. I'd be happy to provide some information.

Although there isn't an option to add a note to invoices without the customer being bale to see it, you have the option to leave a note in the customers profile. Here's how this can be done:

1. Head to the "Sales" tab and click on "Customers"

2. Locate and select the customer in question

3. Click "Edit" 

4. Scroll down until you see "Notes and attachements"

5. Enter the note you'd like to leave.

6. "Save"

Here's an article for reference.

You can remove/edit this note as much as you'd like. I can see how being able to leave the note directly on the invoice is something you'd like to see, so I recommend leaving feedback. These comments are viewed and considered by our developers as they create features and updates for QuickBooks. 

I hope this helps! If you have any other questions, feel free to reach back out.