There are no restrictions against shipping bp firearms to Michigan. However, several years ago there were some misunderstandings of Michigan's (at the time) registration requirements, and some sutlers proscribed shipments to Michigan as a result. Those registration requirements have since been clarified and greatly reduced. However, some sutlers are still operating under ancient and no longer applicable rules. Good luck getting them to change.
And as long as I'm at it, why anyone would even consider trading with an outfit like Cheaper Than Dirt is beyond me.