Who-are You Frisbey, where I am, registered post can be tracked online on the postal service's website (Royal mail & Australia Post), keep your delivery receipt from when you mailed it, then once it's delivered, take a screenshot from the website...proof of delivery sorted.
Include both in your evidence/exhibits as proof of mailing & proof of delivery ;-)
Printed copy · Anthology Volume 3, page 559Printed copy from The Tender For Law Anthology, Volume 3, page 559 (printed 13 June 2014). Added by this site; not part of the Facebook export.