Landr seems to be the big player in this online mastering business now.

They also have some sort of partnership with CDBaby, I get messages from CDBaby pushing LANDR on a regular basis.

They are supposed to do more than just take a song and compress it.

From the CDBaby site:
"Our powerful cloud-based mastering tool is driven by algorithms that understand the nuances that make every song unique." and "LANDR listens to and understands and applies a custom set of tools and parameters for each song."

I have never tried it myself, but I probably will at some point, why not? Nothing to lose, they give you a free preview, if you like it then you can pay for it.