Considering on how many Platinum points you could earn weekly and how rarely My Nintendo comes with actually worthwhile rewards anyway, 160 Platinum Points shouldn't be too expensive of a deal. That discount will no longer be available after September 2nd.
Here's a few weekly missions you could do to earn a total of 90 Platinum Points:
- Visit nintendo.com with your Nintendo account logged in (30 Platinum Points)
- Click on the platinum coin on the main page of My Nintendo (30 Platinum Points)
- Connect to either 3DS or Wii U eShop with respective hardware (30 Platinum Points)