Amazon, ebay, flipkart, banks are charging 15-18% on the services offered not on the electronic transactions. If electronic transactions were to be taxed, people would stop paying online. So this is not officially added. And the tax percentage are on the service provided.
Here you will be charged GST in the following way.
1. Transaction provider to service provider. e.g. say Payumoney to website owner in 15% GST
2. Customer being charged by the website owner by adding that 15% charges.
Electronic commerce has no rule of 20L so every service provided electronic to Indian customers are charged under 15%. The current IGST for total transaction would be with 3% additional total 18% for the online transactions.