When a Shopify store is closed, no further billing will take place through the Shopify system. Under the assumption that you successfully closed your store (you do not see other charges), and the fact that you know it is a charge for a specific app, then it would be safe to assume you've provided Aftership with your billing information. If it were a Shopify charge, it would say Shopify + invoice number, but would not give you any indication whether it is for a subscription charge, app charge, etc.
What this all means is that you'll want to reach out to Aftership directly (firstname.lastname@example.org). While using their app, I suspect you provided them with your credit card information so they could bill you separately. If you reach out to them, they can help cancel future charges, and review a refund request.