I think you might run into some legal issues if you pay someone to mill it for you. Especially if they don't have a license to manufacture firearms. Once the receiver is milled then it is a firearm.
The point to 80% lowers is that the hobbyist finishes them in their own workshop to avoid having to do the transfer paperwork. If you send it off to be finished it will have to be serial numbered and transfer paper work done. Just buy a stripped lower and do the paperwork it will be cheaper in the long run.