Selling products online without offering proper support to your customers is not a good idea. Plenty of businesses offer live chat functionality on their sites. FluentChat for WordPress is a handy plugin that lets you add such a feature to your website. This is a responsive script that lets you manage multiple active conversations.
This live chat plugin allows for sharing files and screenshots. It also lets you add notes to customer info. It is ready to be translated and supports custom CSS. You have the option to use preset responses to engage customers. Webmasters will have access to chat logs in the back-end.
This is a commercial plugin from nCrafts. More information is available here.