I am aware that Word is the best tool for mail merge, but it is not the only one. OpenOffice can do it too. Today, I believe that the term 'mail merge' is more related to MailChimp and Gmail than Word. Unfortunately, none of them can solve my problem. Anyway, I found this free web app called DocBlender that supports .TXT templates, but doesn't have API support (although they mention it on their website).