What is causing my high levels of data transfer? (Was: Horrible invoices)

Maybe there's an app installed causing all that traffic? For my part, I haven't got near my 300M limit during the last months. Jolla's data counter says I have received 150M, (but it doesn't say since when)....

Phone bills are matter of user and operator, not device. If your phone bills are exceeding your expectations there are two ways to decrease them: avoid using internet when no free wlan/wifi is available, or compare different operators' invoicing principles against your current usage. Synchronising emails require data transfer, which is not covered in fixed packages. (edit answer to comment)

S/he isn't asking about operator rates, that was background. The sentences with question marks are asking about what the reasons are for driving traffic on a Jolla. S/he would presumably appreciate an answer on methods to find out what is driving traffic in his/her specific case if a general answer is unknown.

In that case I misinterpreted the question. From what I have experienced there are people who are not familiar with the common invoicing principles and the usage of smartphones. It is not always clear how does email synchronising or simple app usage affect phone bill.

Hey op: Could you please edit the title to something more meaningful? 'Horrible invoices' gives no clue what is your post/issue about. Thanks a lot...