Beware the phishermen!
All online spends are reversible, even when the banks claims otherwise on their websites!
In my long experience I have seen a dozen Canadian exchangers blown out of the water using this model.
Better raise your fee from 5% to 25%. That way when only 1 in 4 of your customers is spending from an account they don't own you'll be breaking even!
Are you referring to email money transfers using certapay? I am pretty sure that these can not be reversed, in fact I tried to have one reversed for tickets I bought online and the seller then never emailed me the money and the bank could do nothing.
Here is a quote from their website:
"Always be careful when sending money
An Interac e-Transfer transaction cannot be reversed once the recipient of the funds has deposited the transfer. You must obtain a refund directly from the recipient. You can ask the recipient to send you an Interac e-Transfer for the refund amount."
http://www.interac.ca/index.php/en/interac-etransfer/etransfer-detail