Vivek Gupta, Tripti Sagar, Dr. Chetan Panse. “Use of Online Payment Transactions across Tier-1, Tier-2 and Tier-3 Cities in India”. International Journal of Advanced Science and Technology, Vol. 29, no. 4s, June 2020, pp. 3274-82, http://sersc.org/journals/index.php/IJAST/article/view/22720.