Motivated by the collection outsourcing phenomena under Extended Producer Responsibility (EPR), this paper studies a contract design problem for a manufacturer who consigns the used product collection to a collector, while the manufacturer only has incomplete information on the collector's cost. On the basis of the incentive theory, optimal contracts are developed to minimize the cost and satisfy the collection constraints prescribed by EPR. Properties of the contract parameters are derived, and issues such as information rent and information value are also explored. The impacts of EPR are analysed by comparing whether or not EPR law is implemented, and more managerial insights are further obtained through numerical examples.