Unfortunately, using the service isn't entirely free, but the fees are well within reason. Registering an account is free, and so is making a payment when you use your iDebit balance. If you make a payment through your online banking, a fee of C$1.50 will be charged, and it costs C$2 to request a payment from your iDebit balance back to your bank account.
Receiving money from the merchant or casino to your iDebit account is also free of charge. This means, that if you want to keep the fees for using the service as low as possible, you can keep balance that you received in the account and spend it elsewhere. Remember, these fees may change in the meantime, you can find an overview of the fees on their website.
Also, keep in mind that online casinos may charge their own fees for deposits all withdrawals, although most casinos cover the charges for their users.