There should not be an additional cost for anyone. What pricing teir do you have?
Im not sure if the API will allow any domain ordered to have Managed DNS, but Ive kicked the question over to Tucows and Ill put an update in this thread as soon as I have a response.
What exactly do you need help with? This is an Older Thread, but if you describe your problem thoroughly then I will do what I can in order to help you resolve your issue.
Hope I can be of assistance.
Reached 5000 Credits! The highest of any member on Smokyhosts! New milestone for Me!