Adding HTML / JavaScript to the sidebar

Hello,

I’d like to add a Shopify Lite button to the sidebar of book pages under the book image. The button is a JS and can be seen on the book page link provided (button is orange).

It would be nice to do it without modifying the plugin files so there won’t be a problem with plugin updates. However, it doesn’t seem like the plugin is prioritized to have a major updated any time soon, so I’m less concerned about that (will backup files).

Thank you so much for any help and hints.

–Andrew
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Highly Optional Reading:
If anyone wonders “Why Shopify button?” here is some context below:

I do like the theme and plugin and hope to make things work. I’m doing this work for free for a jazz pianist who is generous with his time and making great content. In exchange it’s good experience for me. I would like to implement WooCommerce (like his current site is using) at some point but will need to learn how to implement it in a very minimalist way for a digital product. His current site makes it too time-consuming for people to buy courses by having lots of unnecessary form fields to complete. Additionally, he’s not able to update many plugins (including WooCommerce) without breaking the transaction process; so I’m not sure what the previous developer did to cause that. So the Shopify Lite button seems to be a good low cost solution for now.

The page I need help with: [log in to see the link]