By considering the store-assistance service on reducing the rate of consumer return, the differential game model is established, and the problems of the store assistance decision and channel coordination are studied. Firstly, the optimal strategies of the retail price and store assistance are given in the decentralized and centralized decisions by applying the differential game theory. Also, the comparison analysis concerning on the optimal strategies is given in two decision patterns. Then, a two-part tariff contract is designed to increase the profit under the decentralized decision, and coordinate the supply chain in the dynamic setting. Finally, a numerical example is given to analyze the effects of the efficiency of the store assistance effort and store-assistance level on the optimal solutions and coordination. It is shown that the store assistance effort under the centralized decision is higher than the one under the decentralized decision, and the relationship of retail prices under two decision patterns depends on the system parameters. In addition, the coordination capability of the two-part tariff contract is improved by increasing the store-assistance effort effectiveness or the store-assistance level effectiveness.