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
Superbit: I'd strongly recommend being careful. I recently sold <0.5 BTC via e-mail money transfer, received, everything was good. A day or two later, debit card and online banking stops working -- I'm apparently under federal / criminal fraud investigation because the person who's bank account it came from is disputing the transaction. Next thing you know, and going against everything I've been told or read on the bank's and interac's website, my bank is reversing the transaction.
Although they may not be reversible directly, my bank is taking it upon themselves to take my monies and send it to the 'victim', thereby making me the new victim.
Oh, and it's RBC that's giving me problems btw. Waiting to hear from their elevated customer service team...
Good luck!