GA4 purchase event firing at order placement not delivery
COD orders are counted as revenue in GA4 immediately at placement. A COD non-delivery rate of 15-25% means the GA4 revenue figure is overstated by that proportion. Your ROAS calculation for campaigns targeting COD buyers is built on inflated revenue numbers. A second GA4 conversion event at fulfilment confirmation is the correct measurement approach: the placement event records intent, the fulfilment event records actual revenue. Google Ads campaign optimisation should be configured against the fulfilment event, not the placement event.