Invoices and receipts are automatically generated from a template that is stored in Salesforce. If payment terms change, you can update the template using the following instructions
Click on your name in the top right corner of Salesforce
Choose "My Settings"
Select "Email" --> "Email Templates" from the left side menu
Choose the Executive Education Folder
Select your template by clicking on its name
Invoice: Exec Ed Invoice
Receipt: Exec Ed Payment Receipt
Once the template is open, click "Edit HTML" (the text template can be left blank)
IMPORTANT - don't exit the "secure payment link" ...just edit text around it
Use the Send Test and Verify Merge Fields button to preview the template, be sure to choose an existing contact with a related opportunity so you can see how the template is filled out.