Ava here from Shopify.
Once your store is closed, you would not be charged again. In order to close the store, you may need to pay for any outstanding fees. App charges, shipping and transaction fees must be paid in order for the shop to be successfully closed. You can see these fees in the admin under The internal dash invoice doesn’t break down where these charges are from, so accessing the invoice under Settings > Billing > Bills > View upcoming bill. After those fees have been paid and your store is closed, you will not be charged again for that Shopify store.
Let me know if you have any more questions, happy to help!