Ted here from Shopify, thanks for posting your query!
After you select a plan, you won't be charged for anything until your free trial ends. When was your most recent invoice issued? A great breakdown of how Shopify charges can be found here if you would like to take a look.
When you cancel/close a Shopify store; this will automatically stop your monthly subscription and close the account. You can cancel or pause your store as mentioned in this guide. There is a Pause plan which would allow you to place the account on hold for 3 months if that suits you better? I'd always recommend saving your store's data to another device as mentioned here before making any changes, just so you have this data stored for future again if needed.
Can I ask why you would like to close the store? How have you found the platform so far? I'd be happy to pass along your feedback or help you with your store's set up if you would like.
Thanks for getting back to me! I'd be happy to take a closer look into this with you. To do so, I've sent you a separate private email to help authenticate the account. Please reply back to my email when you have a moment.
If there is anything else I can help you with, please let me know, I'm happy to help!