Summary
If the Work Order email needs to also include the Invoice, go to Settings/Work Orders and put a check mark in [X] Attach invoice to completed work order email.
Key Points
Update the Attach invoice to completed work order email setting
Disable the Automatically Email Invoices setting in the customer's Billing page
Click the Settings gear icon in the upper-right corner
Click Work Orders
Click [X] Attach invoice to completed work order email
Note If you have Automation Type set to Automatically Email Invoices in the customer's Billing page, the customer will receive two emails. One with the Work Order and Invoice and one with just the Invoice. If you go with [X] Attach invoice to completed work order email, turn off Automatically Email Invoices.
Also, when you are creating the Work Order, the billing frequency must be marked For Every Work Order. Monthly, Quarterly, and Yearly settings do not send an Invoice with the Work Order.