If I have a giftcard for the Nintendo eshop, can I use it to buy Breath of the Wild?
Yes you can. On the e-Shop screen on your Switch you can scroll to the bottom, choose “redeem code” and enter the code on the back of the card to transfer the value to your account. It is also possible to use a gift card during checkout as well.
You can also go to https://ec.nintendo.com/redeem/#/ to redeem a gift card or other code online.