here is the flow.
when user adds a product in his cart, we show him a popup with a form which has the permission to enable messenger messages to that user. the user clicks it and we save his cart items and fb messenger id in the database (separate table), we track the user if he buys or not, or if he removes the products.
You need to save the messenger id along with product id that has been added to cart. If user buys the product, we will have to add something in table to show that he bought, if he left then we we add that abandoned cart.
Should be delivered in ocmod file type for easy installation. Should be compatible with OC V184.108.40.206
Theme used is Journal Opencart theme.
the popup should use facebook checkbox plugin within a form. the popup can be simple like showing the products added to cart along with the checkbox to connect with messenger. like given in facebook checkbox plugin example.