we need to send personalized emails to our clients, including specific information like ZIP Code, etc. I think we can achieve this using Custom Fields, is this right?
We also need to send these emails "on demand", when the client clicks a button in our website, so we can't group the recipients in a list, and send multiple emails through a campaign.
Can we do this using the CM API?
Thanks for your question, but i'm afraid that we currently don't allow for single sends through the API. Personalisation through custom fields is an important feature of ours, but at this stage we don't have a paid model for single sends, and anything else is simply too open to abuse.
It's definitely something we're considering, however, and we'll add your voice to the feature request.
Can't you use a modified version of the subscribe form for this as it does send a confirmation reply?