An important note in addition to this guide, is that if you SEND money to an artist, should he/she not send an invoice, is that unless you are getting an actual physical commission, such a traditional art, plushie, etc, you are purchasing a SERVICE from the artist, not GOODS. With digital art and stories, you're paying them for their time and service, not for the creation of a 3-D object. Paying someone to write a story is a SERVICE, paying someone to make a physical book for you is purchasing a GOOD.
Not mine, ganking this from RedBeanViolin over on FA, because it's useful as hell for all comissioners/comissionee's.
How to use Paypal for comissions: