It is for $9 in Woolies, so this seems to be a 'better than half' price promo. No minimum quantity restriction. Amazon is probably price-matching Chemist Warehouse so the credit should go to the latter and one should purchase from them if convenient.
$2.70 is a good price for a 200g toothpaste anyway.
I tried other oral-b toothpastes (gum, enamel protection) and despite what some reviewers say I did not experience any mouth burn. Was unable to find any professional reviews, unfortunately. Don't know which one is objectively the best as per scientific evidence: this, pronamel or anything else.