Commercial electric vehicles (EVs) have increasingly gained interest from urban freight companies in the past decade due to the introduction of economic and policy drivers. Although these factors promote urban freight electrification, some barriers hinder the transition to fully electric fleets, such as the significant monetary investment required to replace the current internal combustion engine vehicles (ICEV) and the lack of readily available electric freight vehicles. Due to these barriers, for the foreseeable future, urban freight companies will operate mixed fleets with a combination of EVs and ICEVs to balance their cost/benefit trade-offs. This intermediate operational stage will allow companies to adjust their operations, test EVs, and decide if a fully electric fleet is the best choice. This paper focuses on urban last-mile deliveries in the USA and proposes a long-term planning model to explore the effects of external factors (i.e., fuel costs) on planning decisions (i.e., EV share) for a mixed fleet. In the context of this paper long-term planning is the planning for the infrastructure needed for the introduction of EVs (i.e., fleet composition and charging station location). The goal of the proposed model is to minimize the fuel, EV, ICEV, and EV charger costs. The results show that the EV share of a mixed fleet is affected by gasoline and electricity prices and the distances traveled in a given network. This paper shows that the EV share of a mixed fleet increases when the gasoline cost increases and the electricity cost decreases.