How to cancel the subscription?

Uninstall Power BI Connector for Shopify. You can easily do it by following steps provided here.

Be sure to consider app billing cycles when planning to uninstall an app. Recurring app charges are generated when an app charge is first approved and then again on the first day of each billing cycle. As a result, a charge may still appear on your bill even if you uninstall the app a day or two after a new billing cycle has started.