Crowdshipping has grown in popularity as a sharing economy model, but ensuring its sustainability remains a challenge. This study explores how public transit riders can be engaged in crowdshipping services to avoid generating additional motorized traffic. The propensity of public transit users to participate in crowdshipping and their responses to alternative task attributes are explored through an in-person survey conducted at selected subway stations in Stockholm. The influence of different socio-demographic factors and trip features on the propensity for participation is examined using statistical analysis and regression models. To quantify the trade-offs among required detours, compensation, and parcel weight when accepting crowdshipping tasks, alternative discrete choice models are investigated. The results reveal that factors such as age, employment, and income, along with trip characteristics, significantly affect participation propensity. The estimated willingness to work as a crowdshipper aligns with previous studies showing that age and income level were important factors. A latent class model further reveals a clear division between two groups: one younger, lower income group with higher willingness to work, and another older, higher-income group with lower willingness. As a result, dedicated strategies need to be considered by future crowdshipping service providers and policymakers.