The trading fee is calculated by an algorithm and is dependent on a number of factors like for example volume and order size. Normally the fee is between 0,4 and 2,5%, however with very low volume cryptocurrencies the fee can be higher. With instant orders you will always be notified beforehand when this is the case.
For limit orders it is impossible to say beforehand whether a higher fee will be used. Therefor it might be that you won't get notified upfront. By activating the limit order the user agrees with this condition.