Thats unfortunate, the extension developer is Woocommerce which is what Jevelin is being advertised to be compatible with, I would assume an extension would work with it. They have already said it’s a theme issue, which seems to be true as it works on other themes with all the plugins still activated.
If you scroll down to the Buy Now section on the main page, you will find the One Page Checkout Extension. If you try to add a quantity of product, the cart underneath it will not update. But, if you go to the product page (which I don’t want customers to do) and then add the product to the cart, then go back to the main page, you will see the cart in the extension is updated.
I’ve deactivated everything minus those plugins and it still doesn’t work. The extension works when the theme is deactivated and all plugins are working, so it seems to be a theme issue.