Thank you for sharing your issue in such detail, @matt-johnfrank. You are on the right track. To ensure the CIS deduction will auto-calculate, let's make sure you have enabled the CIS contractor checkbox for that specific customer. Allow me to share this to you.
To begin, if your customer is a contractor who falls within the Construction Industry Scheme, you need to check the Is CIS Contractor checkbox. This ensures that any invoices issued to this specific customer will automatically include the CIS deduction.
Here's how:
- Navigate to the Customers and goals menu.
- Select Customers tab and then click on the customer to whom you want to apply the CIS.

- Choose Edit.

- Proceed to More Info and tick the box.

By following these steps, you should see the CIS deductions being applied automatically to the invoices.
Moreover, I'll leave this article here for your future use, which will help you learn how to record a payment from a customer: Record invoice payments.
For more questions about CIS transactions, you can always visit us here in the Community for assistance. You have a great day!