Hello,
Provide me the link to plugin that you use to add that option. Are you sure that that plugin offers the calculator on the single product page? Does it work with the default WooCommerce theme – Storefront?
If you are looking for shipping rules based on weight then you may draw your attention to https://woocommerce.com/products/table-rate-shipping/ or similar. But I’m not sure that this plugin has the option to show the calculator on the single product page, most likely on the cart/checkout only.
Actually, in your case, this question should be related to WC plugin developers more than to theme author because such plugins should be compatible with WooCommerce. If it’s compatible then our theme will support it or we’ll be able to add this support.
Regards