This woocommerce based plugin helps you make flat shipping free for users having specific quantity of orders in their cart.

Alternative WooCommerce “Free Shipping” and “Flat Rate” methods that allows the shop owner to select and make unavailable any other shipping methods on the same zone when this one is available

Set seperate flat-rate shipping costs for both Domestic and International shipping on a per product basis in WooCommerce.

This WordPress plugin adds a tiered flat rate shipping option for the WooCommerce plugin.