Free Shipping when One Item in Cart - https://www.damiencarbery.com/2018/05/free-shipping-when-one-item-in-cart/

free-shipping-for-single-product-in-cart.php

<?php
/*
Plugin Name: Free Shipping by Cart Quantity
Plugin URI: https://www.damiencarbery.com/2018/05/free-shipping-when-one-item-in-cart/
Description: Apply a free shipping coupon when the cart contains only one item.
Author: Damien Carbery
Author URI: https://www.damiencarbery.com
Version: 0.1
*/


class FreeShippingByCartQuantity {
	private $coupon_code;
	private $coupon_description;
	
	public function __construct() {
		$this->coupon_code = 'free_for_one_item';
		$this->coupon_description = 'Free shipping for when the cart has a single item.';

		$this->init();
	}

	
	public function init() {
		register_activation_hook( __FILE__, array( $this, 'create_free_for_one_coupon' ) );
		
		add_filter( 'woocommerce_package_rates', array( $this, 'unset_shipping_when_free_is_available' ), 10, 2 );
		add_action( 'woocommerce_before_cart', array( $this, 'check_cart_and_apply_coupon' ) );
		add_filter( 'woocommerce_cart_totals_coupon_label', array( $this, 'hide_coupon_code' ), 10, 2 );
		add_filter( 'woocommerce_cart_totals_coupon_html', array( $this, 'change_coupon_html' ), 10, 3 );
	}


	// Create the coupon when the plugin is activated.
	// Based on: https://docs.woocommerce.com/document/create-a-coupon-programatically/
	public function create_free_for_one_coupon() {
		// Do not create a duplicate coupon.
		$id = wc_get_coupon_id_by_code( $this->coupon_code );
		if ( 0 != $id ) return;

		$coupon = array(
			'post_title' => $this->coupon_code,
			'post_content' => '',
			'post_excerpt' => $this->coupon_description,
			'post_status' => 'publish',
			'post_author' => 1,
			'post_type' => 'shop_coupon'
		);
						
		$new_coupon_id = wp_insert_post( $coupon );
		
		// Add meta
		update_post_meta( $new_coupon_id, 'free_shipping', 'yes' );
		update_post_meta( $new_coupon_id, 'discount_type', 'fixed_cart' );
		update_post_meta( $new_coupon_id, 'coupon_amount', 0 );
		update_post_meta( $new_coupon_id, 'individual_use', 'no' );
		update_post_meta( $new_coupon_id, 'product_ids', '' );
		update_post_meta( $new_coupon_id, 'exclude_product_ids', '' );
		update_post_meta( $new_coupon_id, 'usage_limit', '' );
		update_post_meta( $new_coupon_id, 'expiry_date', '' );
		update_post_meta( $new_coupon_id, 'apply_before_tax', 'no' );
	}


	// Check whether free shipping is available for the current cart.
	// Enhanced version of: https://businessbloomer.com/woocommerce-hide-shipping-options-free-shipping-available/  
	public function unset_shipping_when_free_is_available( $rates, $package ) {
		$free_shipping_available = false;

		// Determine whether free shipping is available.
		foreach ( array_keys( $rates ) as $rate_key ) {
			if ( 0 === strpos( $rate_key, 'free_shipping' ) ) {
				$free_shipping_available = true;
				//error_log( 'Free shipping is available:' . $rate_key );
			}
		}
		
		// Hide other methods if free shipping available.
		if ( true == $free_shipping_available ) {
			foreach ( array_keys( $rates ) as $rate_key ) {
				if ( false === strpos( $rate_key, 'free_shipping' ) ) {
					unset( $rates[ $rate_key ] );
				}
			}
		}

		return $rates;
	}


	// Add the coupon if there is a single item in the cart. Otherwise remove the coupon.
	// Inspired by: https://businessbloomer.com/woocommerce-apply-coupon-programmatically-product-cart/
	public function check_cart_and_apply_coupon() {
		//error_log( "Cart contents count: " . WC()->cart->get_cart_contents_count() );

		if ( WC()->cart->get_cart_contents_count() == 1 ) {
			//error_log( 'Coupon applied (going to add)? ' . var_export( WC()->cart->has_discount( $this->coupon_code ), true ) );
			if ( ! WC()->cart->has_discount( $this->coupon_code ) ) {
				//error_log( 'Apply coupon' );
				WC()->cart->apply_coupon( $this->coupon_code );
			}
			else {
				//error_log( 'Skip applying coupon' );
			}
			// This CSS can be used to hide the table row with the 'Coupon: Free Shipping!' text.
			if ( true ) {  // Can be disabled by changing 'true' to 'false'
?>
<style>tr.cart-discount.coupon-<?php echo esc_attr( sanitize_title( $this->coupon_code ) ) ?> { display: none; }</style>
<?php
			}
		}
		else {
			//error_log( 'Coupon applied (about to remove)? ' . var_export( WC()->cart->has_discount( $this->coupon_code ), true ) );
			if ( WC()->cart->has_discount( $this->coupon_code ) ) {
				// Remove coupon.
				//error_log( 'Remove coupon' );
				WC()->cart->remove_coupon( $this->coupon_code );
			}
			else {
				//error_log( 'Skip removing coupon' );
			}
		}
		
	}


	// Change the text in the left column where it shows the coupon code.
	// Based on: https://businessbloomer.com/woocommerce-hide-coupon-code-cartcheckout-page/
	public function hide_coupon_code( $coupon_label, $coupon ) {
		// Change from 'Coupon: free_for_one_item'
		if ( 'free_for_one_item' == $coupon->get_code() ) {
			echo 'Coupon';
		}
	}


	// Change text describing the coupon.
	public function change_coupon_html( $coupon_html, $coupon, $discount_amount_html ) {
		//error_log( "Coupon html: " . var_export( $coupon->get_code(), true ) );
		// Change from: 'Free shipping coupon [Remove]'
		if ( 'free_for_one_item' == $coupon->get_code() ) {
			return 'Free shipping!';
		}
		return $coupon_html;
	}
}

$FreeShippingByCartQuantity = new FreeShippingByCartQuantity();