Can I make clients pay before they can download?

Yes. Set up payouts, then in the gallery’s Settings tick "Let the client pay to release the originals" and set a price. Paying by card unlocks the gallery immediately.

Plans: Studio and Studio Plus, with payouts set up.

Your client sees a Pay £… and unlock button and gets a receipt. An invoice is created for you automatically. You can also tie an ordinary invoice to a gallery and tick Release the gallery when this invoice is paid.