Mastering: Level Matching
You should level match your original track against the mastered version. This is the only way to truly evaluate your results. Since always our ears are drawn to the louder version, the true test of how better or worse the mastered version provided - is through level matching. Don't be surprised, when level-matched, that you might prefer your original track rather than the cheap online mastering service you have utilized!
There is a direct inverse relationship between loudness and dynamics. The louder the mix, the smaller the dynamic range of your track. So you need to find a balance/compromise on where to stop.