If you missed out on this sale which is now expired at The Market, or didn't want to buy in bulk in order to qualify for shipping, you can ask the Warehouse to price beat Chemist Warehouse which brings each pack down to $15.29, so $0.30 more than the previous deal but you can then use the Warehouse Free Shipping codes.
Note that the four pack is still available on The Market at $59.99/4 ie $15 each but as discussed in the comments they often go on sale at Smiths City and can price beat them down to $8.50 so this is a good way to keep yourself in stock until a better deal becomes available.