This paper exploited ultra-low cost, commodity, and radio frequency identification (RFID) tags as elements of a reconfigurable intelligent surface (RIS). The batteryless tags exploited in this work are powered and controlled by a software-defined radio (SDR) reader to assist a source-destination link operating at a different carrier frequency. Generally speaking, a near-field, squared-shaped RIS can offer limited gains. However, signal models include small-scale and large-scale fading, direct link, as well as specific parameters relevant to reflection radio, such as antenna structural mode and reflection efficiency, are typically overlooked in the existing work. Technically, the authors derived an algorithm that computes the optimal RIS configuration with complexity of O(M logM) in number of elements M. With the proposed algorithm, the authors numerically demonstrated that the performance gains can reach a plateau for constant elements spacing and increasing number of elements, suggesting that the weak, passive nature of backscattered links limits the performance gains, even with perfect channel estimation. The authors further studied issues relevant to CSI estimation, including the linear minimum mean squared error estimator and its performance. This work also demonstrated a RIS prototype that is complexly wireless and batteryless, using commercial RFID tags as the RIS elements.