Why does it need my total sales figure?

ACOS divides advertising spend by ad-attributed sales. It describes advertising efficiency, but it is not a complete measure of business profitability.

TACOS divides advertising spend by total sales, including organic sales. AutoPPC supports this when you supply seller total-sales data for the matching marketplace, currency and date window. Without a valid denominator, it does not invent a TACOS value.

A falling TACOS does not prove ads caused organic growth. Price changes, seasonality, stock and non-ad demand can also change total sales. Read it alongside contribution margin and the underlying sales and spend.