Free shipping calculator for cart and checkout

Block Cart – With AJAX refreshing

add_filter( 'woocommerce_package_rates', function( $rates, $cart_items ) {

	// Skip When Free Shipping Is Available
	foreach( $rates as $rate_id => $rate ) {
		if( 'free_shipping' === $rate->method_id ) {
			return $rates;
		}
	}

	// Get Cart Subtotal
	$subtotal = (float) WC()->cart->get_subtotal();

	// Handle Free Shipping Threshold Reached
	if( $subtotal >= 100 ) {
		return $rates;
	}

	// Calculate Remainder
	$remaining = 100 - $subtotal;
	$remaining = sprintf(
		'%s remaining to get free shipping on this order.',
		html_entity_decode( strip_tags( wc_price( $remaining ) ) )
	);

	foreach( $rates as $rate_id => &$rate ) {
		if( 'flat_rate' === $rate->method_id ) {
			$rate->delivery_time = $remaining;
		}
	}

	return $rates;

}, 10, 2 );

Block Cart or Checkout – Non AJAX

add_filter( 'render_block', function( $block_content, $block ) {

	// Cart or Checkout Shipping Block Only
	if(
		$block['blockName'] !== 'woocommerce/cart-order-summary-shipping-block'
		&& $block['blockName'] !== 'woocommerce/checkout-order-summary-shipping-block'
	) {
		return $block_content;
	}

	// Handle Null Cart
	if( empty( WC()->cart ) ) {
		return $block_content;
	}

	// Get Cart Subtotal
	$subtotal = (float) WC()->cart->get_subtotal();

	// Handle Free Shipping Threshold Reached
	if( $subtotal >= 100 ) {
		return $block_content;
	}

	// Calculate Remainder
	$remaining = 100 - $subtotal;

	// Output Message
	return $block_content . sprintf(
		'
			<div class="woocommerce-info">
				<strong>%s remaining to get free shipping on this order.</strong>
				<a class="button" href="/shop/">Shop Now</a>
			</div>
		',
		wc_price( $remaining )
	);

}, 10, 2 );

Classic cart and Checkout

add_action( 'woocommerce_before_cart', function() {

	do_action( 'ccom_free_shipping_calc' );

} );

add_action( 'woocommerce_before_checkout_form', function() {

	do_action( 'ccom_free_shipping_calc' );

} );

add_action( 'ccom_free_shipping_calc', function() {

	// Get Subtotal
	$subtotal = WC()->cart->get_subtotal();

	// Handle Free Shipping Threshold Reached
	if( $subtotal >= 100 ) {
		return;
	}

	// Calculate Remainder
	$remaining = 100 - $subtotal;

	// Output Message
	printf(
		'<div class="woocommerce-error"><strong>%s</strong></div>',
		wc_price( $remaining )
			. ' remaining to get free shipping on this order.'
	);

} );