I have just discovered this. I always thought that the new Nintendo Rewards GoldCoins is only for digital copies. Didn't realized you can claim rewards from GameCards you bought earlier!
Here's what you do:
1. Insert GameCard.
2. Highlight the game from the Menu
3. Press "+" (option)
4. Select "MyNintendo Rewards".
5. Select "Earn Points", then choose the eshop account you want to put the coins to.
I have collected 60 coins each from Mario+Radbbid, Mario Oddysey and Splatoon 2. (Mario Rabbid i bought 2nd hand somemore...lol)
The info there says it's applicable for games released within a year. So quickly claim now! Probably games like BOTW cannot claim anymore since it was a launch game over a year ago. Those who have a physical copy of BOTW maybe can try and update here!
I was also thnking....does it checks for unique serial no in the Gamecard? If not, we can just pool all the gamecards, share and plug in to our Switches to claim reward for each individfual titles! LOL.
Too bad digital copies bought previously can't claim....
NS owner: Quickly claim your GameCard GoldCoins!