Mr Price - Online promotions complaint

Good day
I would like to lay a complaint against Mr Price. Few weeks back they had a buy 1 get 1 free special, i then ordered a bag on their online thinking i was going to get 2 bags since it was a buy 1 get 1 free.
When i went to collect my order i find that they delivered only 1 bag instead of 2 as per the special. Customer care line was called and i was advised that i was supposed to add 2 bags to my basket before paying. Mostly from other online stores the second free item gets added automatically and that's whats i thought its going to happen