Choose the Nearest location rule. It only considers locations that can fill the whole order, then picks the closest to the shipping postcode. You need no paid maps key.
- With coordinates and Free geocoding on: real distance. Shipping postcodes are looked up through OpenStreetMap Nominatim and cached. This service is off by default and sends only postcode and country for orders.
- Otherwise: it compares postcodes in the same country, which works well within one country and makes no outside call.
- If the service is down: the order is still allocated. A lookup waits at most 3 seconds.