Kindly welcome to the Shopify Community.
I properly read your problem and need to carefully analyze your website to provide a solution here.
Would you kindly share your website URL and if your website is password protected then also provide a password?
I will carefully check out the issue and provide a solution to you here.
Please let me know if you have any further questions.
- your store URL;
- page URL with the issue you mention;
- storefront password (if your store has one).
If the store is not online yet, please follow this quick tutorial to learn how to safely and temporarily share an offline/unpublished theme URL.
You're using an external app that modified the "add to cart" - it seems to be some sort of upsell app, I am not sure. But I can guarantee it's coming from an app.
Find out which app it is and urgently contact the app developers to provide a solution because this is a critical bug.
In the meantime, in case your store is already up and running with clients visiting it, uninstall/deactivate the app to ensure it works properly. If necessary, install a fresh version of the Supply theme.