Send Transactions Email to Customers only with Invoices

1.    Navigate to Lists > Relationships > Customers

2.    Edit any customer

3.    In the Preferences tab > uncheck Email next to Send Transactions Via

4.    Click Save

5.    Customize your preferred Invoice Form

6.    Screen fields tab > Messages subtab

7.    In the To be E-mailed, mark the Default Checked box

8.    Click Save