Recently, consumers have been increasingly shopping due to the development of e-commerce; thus, many traditional firms producing green products are entering e-commerce platforms to sell products for their survival. In the contexts of online sales and carbon tax policy, firms need to determine an optimal carbon reduction level and online return strategies. To address firms’ decision-making challenges, we consider a firm producing and selling its green products via an e-commerce platform. For optimal online return strategies, we find that if the residual value of the returned product is relatively small, the firm should not offer an online return service; otherwise, the firm should offer this service. Moreover, the results show that carbon tax policy is detrimental to the firm and consumers, while increasing the average customer satisfaction rate of the product benefits the firm and consumers. Interestingly, we find that the platform should reduce its referral fee as the unit carbon tax increases.