Hi jpatonsmith. Thanks for reaching out to the Community. I'll be glad to share more information about adding custom invoices to your invoice. At this time, there isn't an option to add a Deduction field which you can enter an amount that'll automatically get deducted/added to the total. However, with QuickBooks, you can add a custom field which shows on the invoice screen and on the printed invoice. By adding a custom field, you'll have to manually calculate the deduction amount from the total. Here's an article which shows you how to add a custom field: How to add custom fields to invoices.
Let me know if you have more questions by commenting below. I'm here to help.
