-We have a high volume of payments with low margins.

Based on this alone, you need to get your own merchant account and gateway. I highly recommend Beanstream in Canada for the gateway. A proper merchant account (interchange plus pricing, either Chase Paymentech or TD are good) will boost your margins. For example, processing a debit card has a much lower interchange rate than a credit card (I don't think the rate is as good as the USA, but its still 1%+ difference).

Beanstream will also give you the option of Interac Online - which is a flat transaction fee between 20-50c.