Use a form with need and the verb in brackets to complete the sentence, describing a completed action that proved unnecessary: We ordered a translation headset for every guest and paid for them, but the talk was delivered in everyone's first language. We ___ so many. (order)

Explication

Needn't have ordered states that the headsets were actually purchased even though the purchase proved unnecessary. Shouldn't have ordered could criticise the completed action, but it does not use the requested need construction or specifically mark absence of necessity.