Nowadays online mastering is a very viable option for the audio post production stage because audio can be sent digitally. All the mastering engineer needs from you besides your input is the final mixes for each of your songs.
First and foremost, ask for a free test master of one of your own songs with that online mastering engineer. Any reputable and on the level engineer will offer this and you need to start with this because you need to know what your music will sound like in their hands. It doesn’t matter if they are one of the biggest names in the game and have thousands of artists to their credit or even have a number of before and after samples on their site. What may sound good for one artist may not sound good for you and that extends to mastering just as it does to the recording and mixing stage.
You need to also specify to your engineer what kind of work you need done. This includes how many tracks you need mastered and whether you will need a file for replication, meaning whether or not you are interested in having a physical record either in vinyl or CD format. If you’re just interested in releasing your music online digitally, extra steps are required if you want a CD or vinyl version printed such as sequencing and data will be written.
Next, make sure that you know what to expect in terms of cost with that engineer. Different engineers charge different rates for their services and they may charge by different systems, as well. Some engineers may charge per song, others will charge per hour, and others may quote you a lump sum before even starting the project.
The latter is ideal because you want to know precisely how much you can expect to owe at the end of that session. More importantly, you need to establish with the engineer whom you go with that there can be edits free of charge if you require them. It’s good to establish as much as possible at the beginning of the process before any work has been completed so that there are no awkward moments at the end because the two of you were on different pages about any specific issues in the mastering of your music.