I have setup the API connection to Mailchimp. I have 20 fields mapping. Out of 20 fields 19 are passing their values. e.g. address to Mailchimp address doesn't work, but choosing text format in Mailchimp and it's working. But there is a hard one which really doens't want to work. The product pricing field with dropdown menu is not able to pass his value to the mailchimp dropdown field. It seems related to the pricing fields, because the standard fields drop down works fine.
Just to let you know about this probable bug.