The ISP (or any other business) should be required by law to list precisely the amount and conditions for all fees that the customer must pay, recurring and non-recurring. Any potential customer must have the opportunity to get this information from them (and doing so should not require an internet connection, since otherwise it is a conflict of interest if the fee is for the internet connection).

Even fees that are mandated by the government should be required to be listed, although the ISP need not say that they are mandated by the government (if they have the same conditions as other fees they charge, the amount can simply be combined with it). If a customer wants to know about the government mandates, they should request such information from the government or from public libraries (and again, internet connection should not be required, since it would be a conflict of interest if the fees are related to internet access).

If they charge a fee which is not listed, then the customer should not be required to pay that fee. The ISP might (at most) cancel the service (although it is unsure if it should be permitted, due to net neutrality), but they cannot sue you or debt collection etc, if it is their own fault.