I'm looking for a service / API that can trigger an e-mail upon a completed transaction that will allow our customers to rate the service on a per transaction basis and then collate and report on the results.

There are loads of survey type offerings, but they're all a bit heavy. I'm looking for something where the end user has to do nothing other click a link within the email to rate their experience.

With all the complex offerings out there this seems so simple, but I've not found anything.

Thanks!