That's the thing mate...you know I've asked myself that question, i've looked for it but never actually read that with my own eyes. It must be written in law (perhaps some other part) that an bill follows and order because it makes logical sense - but hey, it 'sounds' right and it worked that time. I think it rests conceptually on the point that you are a free agent - no one has the authority to order you around - if you fulfill someones order you are entitled to bill for that performance but (i would imagine) only when notice of said billing is given prior to.
Printed copy · Anthology Volume 1, page 271Printed copy from The Tender For Law Anthology, Volume 1, page 271 (printed 14 June 2014). Added by this site; not part of the Facebook export.