Some processing companies include interchange as revenue and some don't. If Square does, then they still have ~$110,000 / day in revenue, and what you're calculating is their profit. But you're still right that they have high variable costs because of interchange and therefore may not be as profitable as one might think with that revenue.
As an example, Heartland includes interchange in their revenue. "Heartland reported $526 million in gross revenues for the quarter... Interchange accounted for $365.2 million of second-quarter revenues."[1]
At Braintree we don't include interchange in our revenue, so our processing volume is higher than Square's, but our revenue is lower.
That's interesting... I wonder why there is no GAAP measure for accounting for pass-through revenue. I can see it both ways (this problem exists for Groupon as well, who counts the full amount of the voucher as revenues even though they pass half of it on.
As an example, Heartland includes interchange in their revenue. "Heartland reported $526 million in gross revenues for the quarter... Interchange accounted for $365.2 million of second-quarter revenues."[1]
At Braintree we don't include interchange in our revenue, so our processing volume is higher than Square's, but our revenue is lower.
[1] http://www.digitaltransactions.net/news/story/3138