How to create a custom widget using django for use on external sites

[Frankline]

I hava a new site that I am working on. The site will have prior agreement with eCommerce sites to include add-ons on their website.

Consider the following example:

My website, ABC.com is targeting ecommerce sites. 

For every ecommerce site that sells product X, I want them to include an add-on that gives buyers the option to purchase service Z if they so desire. 

ABC.com will be communicating with the ecommerce sites through a REST API.

My challenge is how to integrate my service as an add-on into the external ecommerce sites. This I assume will be in the form of a widget, HTML code, or a bit of javascript. Something similar to the attached image from Amazon.com. I'm aiming to make a simple integration with the external sites to avoid having them do too much on their end.

Is there a best practice on how to handle this?