Welcome to Shopify Community. My name is Olivia and I work here.
Thanks for reaching out with your question in our forums. I'd be glad to help you out with this today, in fact, I'd like you to check out this similar forum thread I answered for some context on billing.
As I mentioned in that post, there are four sections you can find on your invoice (Admin > Settings > Billing) to help you understand where additional charges are coming from.
In your case, it sounds as though you have signed up for Annual Billing on your Shopify subscription. The subscription includes the cost of hosting your website, but does not include apps, shipping, or transaction fees.
Please read the full thread for more clarity and do let me know if you still have questions about your bill after that.