Refund on App

Hello, I’m hoping someone can help!

i downloaded an app that you get a free trail on for 7 days but after these 7 days finish you will be charged.

i cancelled the app a few days before my trail expired but I have still been charged, when I inboxed them about this the lady said that they can’t refund me through the app and I have to contact Apple, what department do I need to contact?