Mailchimp’s API can be a bit of a nightmare if some systems are relying on raw encoded form data, while the API often works best with JSON.
Merge vars in the following format are valid via form post: merge_vars[foo][0][bar] but very difficult to convert to a JSON payload on the server side if one needs to modify it slightly.
If you absolutely want to retain the form data and not convert to JSON, just know that you can send a boolean "false" via the existence of a post key with a blank value.
I’m not sure how it’s done for boolean "true".
Yuji Tomita 