The increasing parcel demand is resulting in increasing traffic congestion and emission problems in urban freight system. Crowdshipping service is an innovative logistics solution which envisions using the excess capacity in existing passenger transport to perform the delivery tasks. However, it may bring extra vehicle kilometres travelled due to detours and new trips generated. To address this challenge, most studies focus on designing and evaluating the crowdshipping service for all parcel demand, while one study points out the potentially larger benefits of crowdshipping service for outlier parcels. Therefore, this study aims to investigate the transport impacts of crowdshipping service for outlier parcels, which are defined as the parcels with high environmental impacts. A case study is conducted in The Hague. First, the parcel carbon footprint is calculated to segregate the outlier parcels. Then, a public transport-based crowdshipping delivery scenario is proposed, with parcel lockers at train stations as the transfer points and train travellers as the potential occasional couriers. The simulation results show that outsourcing the outlier parcels to crowdshipping service is beneficial to the transport system and prioritising outlier parcels of logistics service providers with low market shares can achieve more savings in transport and higher service efficiency.