CPA (cost per acquisition)

CPA is ad spend divided by ad-attributed purchases, stated in dollars per order rather than as a ratio. The arithmetic is trivial: CPA = ACoS × AOV = AOV ÷ ROAS, since ACoS and ROAS are reciprocals carrying identical information. One trap in that identity — the AOV has to be ad-attributed sales divided by attributed purchases, not your storewide AOV; drop the storewide number in and the CPA you get is fiction. Dollars per order is the only one of the three you can set against a contribution-margin ceiling: 30% ACoS tells you nothing about whether a $9 unit and a $90 unit are both worth buying at that rate.

Sponsored ads reporting won't compute it for you: the Ads API v3 column reference carries no cost-per-purchase column on any Sponsored Products, Brands or Display report type — the eCPP family and the dozen columns literally named CPA are DSP only. Off DSP, the one place Amazon hands a brand a per-order acquisition cost is the benchmarks report, and it groups by brand and category, never landing on a campaign, ad group or keyword. Everywhere else you divide by hand on a denominator that shifts with attribution window and billing type, so pin the window before comparing two numbers — and budget acquisition on new-to-brand CPA, which SB and SD make you compute yourself and SP does not report at all.