That's what I expected as well, before I tried. I assume Spotify just prefers to make a few big deals instead of lots of small deals. It saves them from having to manage all the individual payments, and probably the whole "is this really your music?" thing.

So the aggregator's fee is simply an entry fee. As you can see from my numbers above, it's not a lot of money. :)

To be fair, the aggregator I use offers a choice between 15% of the revenue, or a fixed yearly cost of $9.99 per release (first year may be slightly more expensive, depending on release size). Since I used to release a trickle of singles as I finished them, the per-release pricing definitely made no sense for me; the 15% thing was the best deal I could find when I started uploading in 2009-2010. Also, back then (not sure about now), some aggregators' terms actually demanded exclusive digital distribution rights, which was a complete non-starter for me: I wanted to put stuff on YouTube and free downloads on my website too.