On the website the deal is "FREE Heineken silver when you spend $25 or more"
However when I placed an order just now I added a case of beer and added 36 Heineken Silvers for free.
Steps worked for me:
1. Add more than $25 worth of beer
2. Go to the cart, there should be a message to add 1 Heineken silver for free
3. Edit Quantity
4. Check the total price, should be unchanged.
Awesome, thanks OP. Picked up my order, the guy working there said it was an error, and that another customer had just been to collect his 36 (I didn't get greedy and went for 24). Legend for letting us have it. Coupled with the $20 cashback, ended up costing me about $8 for 30 bottles of shite beer and a can of decent beer.