Uninstalling the app should cancel the subscription automatically. Shopify takes care of the rest.
Although instead of uninstalling, a better option is switching to the free plan. The free plan is free forever, and we don’t impose a trial period. You can keep the app and use it as long as it’s under 10 usage per month. Even if usage goes over 10, we may disable the widget for that month, but we don’t automatically charge your store.