Where the bill actually lives
Settings, then Billing. App charges sit on your Shopify invoice next to your subscription, because Shopify collects them on behalf of each app and passes the money along. That is convenient, and it is also why nobody notices them: one invoice with a plausible total can hide eleven decisions taken across two years.
The Bills section lists each invoice by date and total. Open one. Every app charge is a line with the app name, the type of charge, and the period it covers. That page, and not your memory of a pricing page, is the source of truth for what you are paying.
Four shapes a charge can take
A recurring app subscription bills every 30 days from the day the trial ended, which is why it never lines up with the first of the month and why two charges can land in one calendar month with nothing wrong.
A usage charge is billed on top of a subscription against a capped amount you approved once. As your volume grows the charge grows quietly until it reaches that cap. A vendor asking you to approve a higher cap is a normal request and a good moment to reread their pricing page.
An annual subscription is charged up front and normally is not prorated if you leave early. It is the cheapest way to pay for an app you are sure about and the most expensive way to pay for one you are not.
A one time charge is exactly that, and it is the one people forget when comparing an app they bought last year against a competitor quoted per month.
The three that surprise people
A trial that converted. The trial ended on a day you were doing something else, and the app has been billing since. Sort your app list by install date and open anything you have not touched in the admin this quarter.
A tier that moved. Plenty of apps price by orders, products, or emails sent. Growth moves you up a tier without any action from you, and the invoice is the only place that gets announced.
A charge for an app you already uninstalled. Uninstalling stops the next period, it does not refund the current one. The line you are staring at may simply be the last one, which is worth confirming before you write to support.
A quarterly habit
Once a quarter, open the last three invoices, list every app charge with its amount, and put a total at the bottom. Compare that total against the same list from the previous quarter. Anything that grew, and anything you cannot immediately explain, earns a look inside the app itself.
Pair this with the app audit worksheet on this hub. The invoice tells you what you pay and the audit tells you what you get, and a sensible decision needs both halves in front of it. TreStack Timer is billed the same way, by Shopify on your Shopify invoice, as one line for one app.