It's only legal if the ISP states them in their package details or Terms and Conditions, so you should check those for any hidden charges. Nine times out of ten it'll be to cover extra charges for paper bills, credit/debit card payments and things like that.
If the ISP fails to state or refuses to inform you about what the charges cover then that would be a different ball game. The easiest solution is to ask your ISP directly.
PS - We haven't had a Supanet review for awhile so would you mind adding a quick one?