@Federico Fiaschi I think you should be able to view the invoice and check the responsible resource that could have caused this billing. If you are still unsure, please raise a support case.
I understand that the UI might be confusing, here is a step by step screen shot of you can select for billing related support cases without any subscription plan for support.
From Azure portal's Help+Support blade, select the Create a support request option and select the service and subscription that is available and hit next.
In the next screen it will ask for additional details. Select the required drop down and hit next where it will display recommended solutions.
If the solutions do not help, just hit the Return to support request button and say the solution did not help.
This should bring you back to the additional details screen where you can provide the required data and submit the case by clicking Next.
Before submission, you will have option to review and submit the support case.
I hope this helps!!