Nintendo Switch Online membership can be bought using Gold Points

INSUBCONTINENT EXCLUSIVE:
reading the Nintendo Switch Online Service FAQ
used when purchasing a membership in the Nintendo eShop
Switch game purchases
Points are earned automatically when downloaded from the Nintendo eShop, however physically purchases from retailers need to be registered
within two years of the official release date of the game in your country.These points can be redeemed for any reward that requires Gold
Points or used to reduce the price of content on the Nintendo eShop
Nintendo Switch online is not one of these products
To purchase membership, you need the total cost equivalent in Gold Points
Nintendo Switch games
half of September.