Clients using platforms such as Mirakl struggle to have orders close due to different courier codes being used on each site. ie Royal Mail uses the code RM on one site, but royalmail on another - unfortunately it is outwith our control what codes clients use on their sites.
MintSoft currently seems unable to allow more than one code for the same courier, which is extremely frustrating.
You can also define External Courier Service Names in Shipping Management - we've had to do this for our Mirakl clients