https://www.gamespot.com/articles/psa-nintendo-switch-onlines-free-trial-will-charge/1100-6461805/

Nintendo's online subscription service for Switch is finally. The service rolled out earlier this week, and you'll now need to have an active subscription in order to play most Switch games online going forward. If you're still undecided about signing up, Nintendo offers a free trial--but like most trial periods, you'll be charged if you forget to cancel.

Switch owners can now sign up for one seven-day free trial of the Switch Online service from the console's Eshop. However, as detailed on Nintendo's website, if you don't disable automatic renewal before the trial period ends, your trial will automatically convert into a one-month subscription, which runs for US $4 / £3.49 / AU $6.