$100 will give you a $110 Gift card
From their email..
The Myer Christmas Club allows you to get more out of your Christmas shopping. Take advantage of our set-and-forget direct debit service and you'll receive a $10 bonus for every $100 you contribute until 1 October so you can relax knowing Myer has you covered this Christmas.
Your savings and the bonus will be issued as a Myer Gift Card in November 2015 and delivered to your nominated delivery address as specified in your membership application or as updated by you. For further details and full terms and conditions of the Myer Christmas Club click (link on website). The Myer Gift Card terms of use will apply to the Myer Gift Card.
Given the above best to schedule the amount you want to get the 10% bonus on, for late September
Must direct debit $20-350 per months, with maximum $4,999/year. If contributing for 3 months until 1 Oct this equates to approx $100 giftcard.
$20 cancellation fee though, unless cancel after Oct 20.
Seems pretty good.