Square loses money on transactions below $6. The reason is for the sake of simplicity they waive the per-transaction fee that Visa/Mastercard charges them and just charge a flat percentage of 2.75%. But even though this is a relatively high percentage, it's not enough to recoup the cost of the per-transaction fee at low transaction sizes.
Take a $2 coffee charged to a Visa credit card. Visa charges Square 1.76% + 6 cents, or 9.5 cents. But at 2.75% Square only collects 5.5 cents.
So even if Starbucks was already paying wholesale Visa/Mastercard rates, they'll save 4 cents or more a cup thanks to Square's "simplicity subsidy".
Now multiply that by a few billion cups. Starbucks saves millions, and Square is happy to pay it to drive their growth into the stratosphere.
Source: http://feefighters.com/blog/can-square-make-money-with-its-n...