From mix to master
An engineer listens along while you mix, and masters the track when you are happy with it: how it goes and what you end up with.
You mix, and a SoundAssist engineer listens along. When you are happy with the mix, the same engineer masters the track. That is the one engineering service SoundAssist offers, at one price per track: the mixing stays yours, the ear and the master are ours.
How it goes
The track gets its own page. You post a version of your mix, and a SoundAssist engineer listens to it in a room SoundAssist built for itself, on ATC SCM50 ASL PRO with dual SCS70 subwoofers. What comes back are notes that land on the exact second, so a remark about the snare stays attached to the snare instead of floating in a chat thread.
Then you go again. You post what you changed, the same engineer hears it next to the last one, and you keep going until the mix sits right. Nobody starts over each time: the ear that met the first version is the ear that meets the sixth, and it remembers what you were reaching for.
When the mix is done
You decide when the mix is finished. Then the engineer who followed it masters it, so the master is made by someone who already knows the track.
What it costs
One amount per track, and it stands on the mastering page, so you know it before you start. There is no list of extras and no hourly rate.
What you end up with
You get the master as a WAV, at the bit depth and sample rate the track needs. You only pay once you're in love with the finished sound.