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.
Ordered on the website 20 minutes ago, going to pickup now, will chuck the 36 Silver's in the work beer fridge thanks OP!