Interac Online appears to have been deprecated by most major Canadian banks in favour of Visa Debit. My understanding is the Visa Debit cards issued by banks like TD and Scotia are dual-mode: transactions will be processed through the Interac network when possible, with the lower Interac fees applying, and through the Visa network otherwise.
I don't know if the merchant fees for Visa Debit are any different to using a Visa credit card.