“Store unlimited data for free and pay only for what you analyse” - unlimited data for free? Not happening. I would really take that off before someone tries to find out limits of your “unlimited” offering.
If you fire
"SELECT SUM(amount) from shop.sales"
and you have 1 billion sales transactions in your table; since the "SUM(amount)" returns a single row, we count it as 1 row selected for billing purposes. And the charge is $10 per million rows selected. Always rounded upwards to the nearest millionth.