Developers pay Artless Devices, the company behind License Zero, commission on each sale. Commission is currently 1% of purchase price for new accounts. See the commission section of the agency terms for specifics.
Developers also pay Stripe for payment processing. As of September 2019, Stripe offers 2.9% plus 30¢ for new accounts.
Depending on the jurisdictions of the buyer and the seller, one or both may owe sales taxes. License Zero can’t do your taxes for you, but it reports standard jurisdiction codes for buyers and sellers, so everyone has the records they need to comply.