Records from the invoice display can be mailed to customers using the standard mailing facility (see page 90).
You can modify the email that is sent to your customer by editing the tmp_invoice.txt file. This email needs to include a link to the shopprojectlogin.asp file on your site (e.g. http://www.vpcart.com/vpasp/shopprojectlogin.asp), the customer full name and email.