I'm not sure what level of expectation you have in regards to quality, but generally you're going to pay a lot for a top-notch job at a professional mastering house.
Most of the online services are legit, but are often-times home based operations by intermediate to experienced engineers, but often without the advantages the professional mastering houses have in regards to acoustic environment and caliber of equipment.
That said, you may or may not be satisfied with the results of an online mastering service and that's typically why they offer to master a song for free, although usually they will only provide you a sample of the mastered song unless you agree to pay for the whole album to be mastered. You generally don't want to enter into a monetary obligation until you're confident in the mastering engineer's competency, so in your exploration of online services, ensure that you are provided the option to preview the services they're offering, in context with your song, before you make any decisions.
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