Ask HN: How are ads handled for email newsletters?

I'm in the early phases of starting an email newsletter (CAN-SPAM compliant, opt-in, of course) and was wondering how ads were handled. You don't have the option of using Google Ads since they won't reach into email for small fry operators.

My hunch is to charge based on views and to collect metrics on clicks. My question is twofold. Are there services into which you can plug for ads? If not, is it reasonable to simply write a Rails app, for example, that registers the click and redirects to the client's web site? (I'm thinking the url for each ad contains an ad ID and possibly a user ID).

I've exhausted my Google-fu on this one. I'm just looking to avoid pitfalls and find existing solutions as much as possible.

Thanks in advance!