Hi, the CC BY-SA 4.0 license allows commercial use, as long as you credit the author and mention the license. The biggest limitation is if you modify the illustrations, you then have to redistribute the modified versions under the same license.
You can fin the CC BY-SA 4.0 license explained here: https://creativecommons.org/licenses/by-sa/4.0/
We’ll be working on more stock art in the months to come, so come check our page from time to time if you’re interested :)