[Vulconizer]

"Mastering" one song isn't what I really call mastering though...

It sounds like what you are wanting to do is send your mix to someone else to finish it for you. I guess you can call it "mastering". I call it the mixing process and others call it pre-mastering. Pre-mastering? Why call it that?

To me, "mastering" is taking a compilation of songs, putting them in a logical order, then doing whatever bit it takes to make that a cohesive unit. It may take a little EQ, compression, level matching, or whatever. It may not. The mastering engineer is also responsible for putting the spaces or cross fades in between songs and making sure it's industry compliant so that the replicating house can duplicate it.

So, to me, anything leading up to the actual process of working on that compilation is not mastering.

Sure, you can send one song, just know that it's not some sort of magical voodoo. It is an extra set of ears that can listen to your mix critically, maybe apply some techniques that may not be available to you due to a knowledge, experience, gear and room difference, and even give you some advice on things to work on. All for a fee.

Who knows? If you did your mix right and he's a good guy, he might send it back without charging you because there is nothing he can do to it. At least, nothing until he has the context of an album's worth of songs to go with it...