Boku doesn't charge users any service fees for making deposits. However, some casinos apply transaction fees to every deposit. Make sure to check this on the casino's banking page before you make a deposit.
The mobile carriers may also charge standard SMS rates for payment authorisation and other messages related to using Boku. If you don't pay your phone bill on time, your mobile carrier will most likely charge you some extra collection fees.
You can find more information in Boku terms of service.