Hi there,
Having the option to add custom fields to your invoices in QuickBooks Online means that you can get creative with the type of information you'd like to share with your customer. QuickBooks Online makes it so easy for you to add custom fields, and it sounds like you're right on track with that. I'll help explain why there's a limit of 3 custom fields that can be added to your invoice forms.
Because of how the system maps your information on to the invoice template, there's a limit of 3 additional custom fields that you can add to your template. I can appreciate that depending on your business, you may need more than 3 custom fields to display your information. At this time, you'll have to write this information in the Message field located at the bottom of the invoice creation screen.
I know how handy it would be to have the option to add more fields to your invoices. I would be happy to submit this feedback to our product development team for review. If you'd like to pass along these suggestions directly, you can do so by clicking the Gear icon > Feedback.
Have a great day.
