Glad to see you here in the Community, @neil-brooks.
To show the invoice number in the reminder email, we need to select the Form name option in the invoice template in QuickBooks Online (QBO).
Here's how:
- Go to the Gear icon.
- Under Your Company, click Custom Form Styles.
- Click the Edit link of the default invoice template.
- Select the Content tab, and click the header part of the invoice to expand more options.
- Under Form, check the Form Numbers box.
- Click Done.
Then, try printing your invoice and check if the invoice number shows up. Here's the link: How to email or print an invoice.
I've also added this article about customizing invoices in QBO. It contains complete information on how to change the layout, design, and data fields that show on the invoices: Customise invoices, estimates, and sales receipts in QBO.
Don't hesitate to drop a comment below if you have other questions. I'm more than happy to help. Take care!
