Rishi Mehta

This plugin activate add to cart through ajax on varible product. By default woocommerce is not having this feature. Plugin is adding own jquery which is differ from woocommerce default add to cart jquery for simple product.
This plugin also supports sending additional field and other data added by other plugins. It will work with most of product addon and extra field data.
Let me know if you stumble upon any plugin conflict in support forum.

Woocommerce Ajax add to cart for variable products

This plugin activate add to cart through ajax on varible product.

WP BackOffice

Link directly to a specific WooCommerce product variation using get variables – yoursite.com/your-single-product/?size=small&color=blue. This is useful if you want to send an email to customers and send them directly to a specific variation without them having to enter it in themselves.
Usage/Examples:
Attach: ‘?[variation_name]=[variation_value]‘ to the end of your product link to have the variations automatically appear on the user’s screen.
<ul>
<li>mysite.com/product/happy-ninja/?color=blue</li>
<li>mysite.com/product/happy-ninja/?color=blue&size=small (additional variations should be separated by ‘&’)</li>
<li>mysite.com/product/happy-ninja/?color=blue+green (where the variation value is “Blue Green” with the space replaced by a ‘+’)</li>
</ul>
Features:
<ul>
<li>No configuration needed, just install and start using</li>
<li>Supports an infinite number of variations</li>
<li>Fails gracefully, if the option or value doesn’t exist, the default value will appear instead</li>
<li>Default selected variations work if no product is specified in url</li>
<li>100% case insensitive</li>
<li>Works with any variation settings you have ie. custom images, prices etc.</li>
</ul>
<a href="http://wpbackoffice.com/plugins/woocommerce-direct-variation-link/" rel="nofollow ugc">Plugin’s Official Documentation and Support Page</a>

WooCommerce Direct Variation Link

Link directly to a specific WooCommerce product variation using get variables (yoursite.com/your-single-product?size=small&color=blue).

Woocommerce Ajax add to cart for variable products vs WooCommerce Direct Variation Link