A mastering engineer typically won't be able to make changes to individual elements in your songs, that is almost always a mixing change. When submitting your mixes keep in mind if you don't like how loud your vocal, guitar or awesome synth is, make sure it's right before you send it in. If you don't and mastering doesn't fix the issue, you'll have to submit a new mix. And every time you do that, it will be considered a brand new order.