If someone requests a new invoice, there's an easy way to do it in Salesforce.
Navigate to the related opportunity
Towards the bottom of the page find the "Activity History" related object. Select "Send an Email."
Select the search icon in the "to" field.
The list the pops up includes your most recently view contacts, so the top result should be the one you are looking for. If not, use the search bar.
The contact name will appear in the "to" field and their email address will appear next to it. If more than one email address is associated with the contact, a drop-down menu will show additional email addresses you can select from.
At the top of the page select "Select Template."
Change the drop-down menu to "Executive Education" and then select "Exec Ed Invoice"
The invoice preview will appear. It pulls the information from the opportunity. You can edit this email if needed, but it is better to adjust the price on the opportunity than on the invoice (See Changing Price on Opportunity). You can change the subject line and add in CC or BCC addresses. Select "Send."