Let's see how FacebookMessage looks like in the OpenCart storefront.
This is the default view of the Messenger chat bubble in your store:
Your customers can now start a conversation with you without leaving your OpenCart website.
All you need to do is respond from your Facebook business page's messages.
Customers will the following controls over their chat with you.
To summarize, you have the Messenger app itself working like it normally does, but within your OpenCart store.