Super Mario 35th Pin Set Restocked in Nintendo Store
If you weren't able to order a Super Mario Bros. 35th Anniversary Pin Set by purchasing Super Mario 3D All-Stars and completing a few My Nintendo missions, you may be in luck, it has been restocked at the Nintendo Online Store in both the US and Canada.
However, there is one major caveat, you do need a reward code to order - unfortunately they were gone within a short time period. At the moment, they don't seem to be providing any more reward codes for redeeming a Super Mario 35th pin set.
The link on the SuperMario35.com website had been linking to the old product page to redeem a pin set. But, it turns out that Nintendo has added a new SKU for the Super Mario 35th pin set. According to the product page, the pins will be shipped in late October 2020.
Nintendo Online Store (USA) - https://store.nintendo.com/super-mario-bros-35th-anniversary-pin-set-1-ships-late-oct-2020.html
Nintendo Online Store (Canada) - https://store.nintendo.ca/super-mario-bros-35th-anniversary-pin-set-1-ships-late-oct-2020.html
One thing to note is that unlike the old Club Nintendo physical rewards, you have to pay for shipping. Shipping rates are $5 USD or $5 CAD (the help page says shipping is $7 in Canada, but $5 is the amount in the shopping cart) for the pin set. It is possible to combine the pin set with other items in the store - including other physical My Nintendo rewards.
Currently, there are a few Animal Crossing rewards available at My Nintendo - two shopping bags for 800 coins each, and a bandana for 600 coins. Given that it's free, and coins are mostly useless, might as well load up on your order since you have to pay for the shipping.