I'm a professional recording/mixing/mastering engineer with many years in the audio design industry.
The file will be delivered in high quality MP3. If you want the uncompressed WAV mastered file, please check gig extras. Do not order this extra if you're sending me an MP3 to work with.
Things to consider:
Make sure the master output isn't clipping. In fact, you should leave
several dbs of headroom (keep the bouncing master level meters well
below the red, ideally at -4 or -5dBs).
compression, limiting or EQ on the master output of your final mix. Only
apply these processes if you feel they are musically essential.
your mix at the highest possible bit depth (ex: 24bit) and sample rate
(ex: 48kHz). This shouldn’t make a huge difference over 16bit / 44.1kHz,
but the more resolution the more precise the mastering will be.