Due to the increase in Web services, many recent studies have been addressing the service selection problem based on non-functional or quality aspects. Our study incorporates combinational use of functionally-equivalent services into the problem to compose an application of higher quality or with additional value. However, when such combinational use is introduced, computational cost for the service selection becomes much higher. In this work, we propose a set of methods that reduce the additional cost for the QoS (Quality of Service)-based service selection considering combinational use. This approach achieves low cost by considering only effective combinations. The experimental results show that it can reduce computational cost regardless of the number of services and whatever their QoS values are while keeping the effectiveness of combinational use.