How easy to accidentially subscribe a video channel?

My Mum uses my FireTV to watch free TV from her bed. I have Amazon Prime but no additional susbscriptions. My Mum is easily scared and usually calls me whenever seeing something unknown, she would never "buy/subscribe" when even the smallest text appears and tries to inform her about something, instead she calls me in utter panic why her TV suddenly acts different.

So two weeks ago she suddenly had subscribed to a local channel "Herzkino" - not expensive but neither did anyone get a receipt nor any other information. It was just €3 so what, cancelled it after the first month.

So now I wonder if her claim "there never was any warning about costs" is correct, how does subscribing to channels usually work? And why didn't I get any receipt? I mean if I buy a €0 app for my FireTV I get like three emails but not for subscribing to a channel?

Funny side note, Herzkino is not even commercial. If you visit the local Mediathek of out public TV station it is 100%, it just costs money watching it through Amazon Prime which is kinda ridicilous because if on the very same FireTV I use the ZDF Mediathek instead of the Amazon Prime app I get everything for free. I have no idea how and why this happened and why Amazon is even offering to subscribe a channel which is totally free just one click to the left.

It is not about the €3 but about the daring moves behind it.