Unknown charges

Fnb is charging me declined fees for transactions that I don’t even know how do they allow a transaction to happen 10 times or 20 times with the same reference.Aren’t they thinking that could be ***** how fro you charge so much charges.I pay a monthly fee but I will have unknown charges.Secondly I buy something and they reserve the money and when they decide to take it and there is no money they charge me unpaid for what because the time I was paying there was money in my account.I feel like FNB is ripping us off with these ridiculous charges and it’s small amount and by the time u check u have so much charges and this has been happening for 3months now.And this is happening even to my son fluid account who is 12years receiving ridiculous charges on his account which is under me.