The type of email you are talking about is called a Transactional Email. This type of one off email is not an email format Constant Contact currently offers. Our emails are generally geared towards group/bulk emails as opposed to single use emails. It is certainly possible to program the API to do this type of single send email, however the email can not be scheduled to run programatically (I.E. you would have to log in and manually schedule these emails) and you would end up with a very large volume of single send emails in your account. It is generally recommended that you have your server that is processing the requests handle these transactional emails immediately during the processing of the purchase.
We will update if we add functionality in the future for this type of email sending via our developer website and developer newsletter. I have also provided feedback to our product management regarding this feature.
Dave Berard
Senior Product Manager, Constant Contact
