Artificii uses the AOI's Model Terms of Trade. It is a contract model - for most flat-fee commissions - that protects the client's interests and the illustrator's interests.
Typically, Artificii grants clients a licence for a particular use for the artwork and the fee charged will vary in line with that use. This gives a client the protection they need for the use of any given image.
You may want a book cover, poster, advertisement etc., your budget may be small or large and your use for any image will vary; so does the pricing therefore.