I don't know what distinction they're implying with those two different mixing categories -- you'll have to ask them.
Whether or not you need for-hire mixing services depends on what your projects are like.. Do you have a finished song with a lot of tracks that need to be balanced and processed? If so, then maybe you might benefit from these guys' services. If you don't have a finished song yet, or if you only have a vocal track, then you don't really have anything that's ready to mix, and it's premature to be worrying about mixing. I don't have any idea what your music projects are like, so I can't advise you on what you might need in that regard.