When you send a recording to be mastered, typically you will have already mixed it. What you export from Cubase should be already be mixed and made the best you can make it. When you export the mixdown it should be at the same bit/samplerate as it was recorded/mixed in, and in uncompressed .wav format. That way you can take advantage of the professional-level dithering and such found at said mastering house. Also, due to the fact that you are new to this (no insult), you should consult the ears and twiddling fingers of someone who is not new to the process before you even consider mastering...You want the money you spend to take you as far as possible. Word?

Word. So then I am exporting a mixdown, and this mixdown is what a mastering engineer works his magic on. I thought (erroneously) that mastering was a track by track deal.
Thanks.