@petra-pik - I just had a similar issue although I didn't notice it until I cancelled the purchase because even though I had paid for the Premium app I was completely unable to access it.

Here's the image showing the original charge amount for a one-year, one time purchase - The payment addition is just US state sales tax shown below:

@slongshit
Same problem, I thought it was great on my phone and went through the "Buy Premium" on my phone but it wouldn't let me access premium on the phone and I can't access it here either, I had to set up this account just to find out why I'm having problems.
Basically them seem to get you to pay for it and then don't let you access it.