Automatic mastering services can be useful if you either already have a good mix and want minimal adjustments, if you simply need a job done real quick, or if you're just looking for a preview, but they're fairly limited as far as taste goes. Usual professional mastering services from a human likely wouldn't get you what you're probably aiming for either tbh. The best place to improve your final output would really be in the mixing stage.
So regarding your mix, my thoughts boil down to:
- Some louder parts are clipping heavily. This should be avoided unless for intentional effect
- Balance is overly warm. you can try a tilt EQ to hear a basic balance that's closer to flat
- Bass/vox could benefit from more dynamic control. This feels more obvious after tilting to a flatter balance
- Vocals have much more presence than the instruments (also more obvious after tilting flatter)
- Vocals and harmonica are too loud compared to the instruments
- Could use some subtle reverb to gel everything together
- Overall mix sounds very monophonic, except for the piano which is actually surprisingly mono-incompatible
Using your source file, here's an example of a balance closer to what you might like to aim for. I used a convoluted heavy-handed processing chain since it's a 2-track source, but essentially tries to address the points listed above.
https://drive.google.com/open?id=1US...a0PTwqMqfxbY7Y