You've come to the right place for mailing your customer's invoices, @customercare2.
To clarify, do you need to send the same invoice to multiple email addresses? If you do, you can add up to three email addresses to the Email field in your customer's profile and separate them by a comma (,).
Navigate to the Sales menu, then choose Customers.
Choose a customer.
Select the CustomerDetails tab, then click Edit.
You can enter the email addresses in the Email field.
Doing this will automatically fill in the customer email field when creating an invoice.