https://www.gamespot.com/articles/you-can-pay-for-nintendo-switch-online-using-my-ni/1100-6461812/

Nintendo's online subscription service for Switch launches on September 18, meaning you'll need to pay to play Switch games like Splatoon 2 and Mario Kart 8 Deluxe online after that date. Naturally, you'll be able to purchase a subscription either through the Eshop or at retailers, but you also have another option for paying for the membership: My Nintendo points.

As is the case with many digital Switch games and DLC, you'll be able to use your My Nintendo Gold points toward the purchase of a Nintendo Switch Online subscription. However, as detailed on Nintendo's support website, the points cannot be used for automatic renewals. You also have the option to pay for a subscription using a PayPal account.