I was checking out Twelfth Planet Press because of a YA anthology call that they're crowd funding. Checking out their guidelines I found that they have a digital novella series open for submissions. They're looking for stories of 10,000 to 40,000 words in the genres of sci-fi, fantasy, horror, and crime. Payment is an advance of $100 to $200 against royalties of 50% of the net. You can check out the guidelines here, just scroll down the page a bit.
And the YA call is for an anthology called "Kaleidoscope". Having reached the half way point in their funding they've opened submissions to the public. They're looking for sci-fi and fantasy stories of 2500 to 10,000 words with a focus on diverse perspectives. Payment is 5cents a word and the deadline is December 31. You can find the details here.