The link provides currency in Euros and Pounds but I can confirm Australia is also having a discount on these titles. This is the first time these titles have been discounted. Normally they're $19.95 for all I believe.
For those who don't know, these are extra games that are available for Streetpass Mii Plaza. For more info, see the official Nintendo webpage.
To purchase, enter the Streetpass Mii Plaza and go to the Shop.
Note: You can only buy the combo pack on the first purchase.
Which platform ???