Hi @JoannaWood !
The most common reason of unknown charges is an active Shopify account that was not properly deactivated. Since this is an account and billing issue, I highly suggest that you reach out to Shopify support team so, they can further investigate where the charge came from. You can reach out to them by going through their Shopify Help Center and enter your concern on AI chat box, respond to it until the “I still need help” or “chat with a support advisor” option appear.