build a website with e-commerce functionality with smooth checkout

We are a young startup called Vikalp based in India developing agricultural tools and equipment and we require a website which is responsive, quick to load and easy to navigate for the user and has an extremely easy to use shop page and seamless checkout page. Our current website is built on wordpress and runs woocommerce plugins to achieve the same but is pretty slow to load and does not always send the order confirmations on time. Please check http://vikalp.tech/ for more details. One special feature that we need is "Risk Free Cash on Delivery" option, wherein, when a customer choses Cash on Delivery as the mode of payment, he is directed to pay a booking amount which is 15% of the cart value. The rest 85% will be payable upon delivery. Needless to say, this same thing needs to be communicated on the email that is sent to the customer who books a COD order which will contain words like "Amount XYZ payable upon delivery"