Currently, the feature to send out invoices that can be edited by the receiver is unavailable. Although, you to inform the receiver to just press the Reply button in the email so that they will be able to make changes. Let us first set up the Preferences in Send Forms section. I'd be glad to show you the steps below.
Here's how:
Go the Edit tab and select Preferences.
Scroll down on the left pane and choose Send Forms tab.
Tick the box that has Use "Plain Text" format for e-mailed invoices.