If you’re enjoying your one-week trial of Nintendo Switch Online, please pay attention.
When you start your trial, an ‘automatic renewal’ option will be active by default. If this option is not cancelled when your trial expires, you will be automatically charged $4 for a one-month subscription.
If you have no intention of committing to the online service now, it is probably a good idea to cancel auto-renewal. Don’t give yourself an unpleasant shock.
If you need help, here’s information from Nintendo’s FAQ:
Can I turn off automatic renewal for a Nintendo Switch Online membership?Yes, you can turn off automatic renewal under Your Subscriptions within the Shop Menu in your Nintendo Account settings or under Your Subscriptions in Account Information on Nintendo eShop on your device at any time up to 48 hours before the end of the subscription term.