Hey, I don't know who this may apply to, but I know that there are a fair amount of commissioners who use paypal on this site so I might as well put this here.

http://dracavy.tumblr.com/post/145416832494/paypal-invoicing-seller-beware

I've never had a problem with it, but apparently a lot of people have. This is probably everywhere already but meh. Here you go anyways.