Thought I would post an update in case others get into the same situation and need a "legal invoice" for their accounting.
After going back forward with their support, I finally got somewhere.
Turns out you need to send a support request to their sales department, asking specifically for an invoice that you can use in your accounting. In addition you need to list what information you want on the invoice, i.e. company name, address, company registration number etc.
A side note here, I was told when sending this request as well, that they only provide the report pages inside their control panel. But I replied saying they are bound by law to provide an invoice, and after that I received one.
One thing is certain, the next time I refill the account I will pay through Paypal, as that allows me to use the receipt from Paypal in the accounting, by just attaching it with a note what its for.