本文建構一個商品通路的模型來解釋線上通路的興起，並藉以探討課徵差別通路稅的福利效果。文中有幾個重要發現：首先，從租稅設計的觀點來看，對於某些商品而言，課徵差別通路稅具有三種福利效果：購物通路的轉換效果、傳統課徵商品稅的 Ramsey 效果，以及所得重分配的效果，而最適稅制即在此三種效果間取得平衡；其次，當線上通路的『平均稅率彈性』不大於傳統通路時，最適的差別通路稅應對較不費時的通路課以較重之稅負。最後，由本文所獲致的結果可延伸出，在忽略網路交易的稽徵成本下，從社會福祉的觀點來看，不僅應對網路交易課稅，而且其稅率應高於傳統臨店交易的稅率。另外，本模型亦可延伸應用於代客泊車、代客驗車及到府維修等省時勞務之消費與提供。

The taxation on commodities has been made complicated by the Internet. The traditional literature on optimal taxation neglects the effects of tax on the distribution channel. This paper attempts to answer the following questions: Should the tax rate be the same both on line and in stores? Should the tax be lower over the Internet? What is the optimal tax structure of the commodity for the different forms of transaction? To our knowledge, taxing different forms of transaction has not yet been theoretically investigated in the literature. The paper establish differences in the purchasing behavior of customers between on-line and in-store transactions based on Becker’s concept of time allocation. With various time costs, the consumer will choose a marketing channel to maximize her utility. In the model, the consumers with the higher wages are likely to purchase the commodity on line.Therefore, the time cost serves as a screening device to reveal the information about their income and provides valuable reference for government policy on income redistribution. To maximize the level of social welfare, the tax rate on on-line purchases is inclined to be higher than that on in-store purchases.