Checking if Shopify Lite embed code is ok to run on Backdrop. The embed code is - script type="text/javascript" plus a div calling the product ID.
I would assume we put the script tag in a js file in the theme's js folder (or in the script[dot]js file itself), and call the product ID from the page, using the Full HTML text format.
Here is their sample embed code from their Help:
Not ready to test. I'm working towards a D7 -> BD migration, where I used Drupal Commerce on D7, and having only one product, Shopify Lite seems a pretty sweet solution.
I took quick look at the page you linked to and it's not clear to me if it would be possible to simply post the embed code into the body field of a node (with the "raw html" text filter). That might be enough.
I may experiment with this a bit myself and will let you know if I do.
Otherwise, be sure to look at this topic:
https://forum.backdropcms.org/forum/e-commerce-solutions-backdropcms
If you only have one product, using the Stripe module (and webform) is also a relatively simple option. https://github.com/backdrop-contrib/stripe. However, we need better documentation for this.