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

Nintendo's online subscription service for Switch is almost here. The service rolls out in the US on September 18 (September 19 in Europe), and after that, you'll 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 will also offer a free trial--but like most trial periods, you'll be charged if you forget to cancel.

Beginning September 18, Switch owners will be able to 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.