Applies a percentage-based import tariff to shipping costs for orders going to the United States. Code based on: https://woocommerce.com/document/add-a-surcharge-to-cart-and-checkout-uses-fees-api/#section-3

add-us-tariff-to-shipping.php

/**
 * Add US tariff surcharge to shipping costs.
 *
 * Custom implementation to apply US tariffs on shipping charges
 * to work alongside the WooCommerce Customs Fees plugin.
 *
 * @since 1.0.0
 * @author Shameem Reza
 */

/**
 * Add US shipping tariff as a surcharge.
 *
 * @since 1.0.0
 * @return void
 */
function add_us_shipping_tariff_surcharge() {
	// Skip in admin unless doing AJAX.
	if ( is_admin() && ! defined( 'DOING_AJAX' ) ) {
		return;
	}

	// Check if WooCommerce is active and cart exists.
	if ( ! function_exists( 'WC' ) || ! WC()->cart ) {
		return;
	}

	// Define target countries and tariff rate.
	$target_countries = array( 'US' );
	$tariff_rate      = 0.25; // 25% tariff - adjust as needed.

	// Get customer shipping country.
	$customer_country = WC()->customer ? WC()->customer->get_shipping_country() : '';

	// Check if tariff applies to this country.
	if ( ! in_array( $customer_country, $target_countries, true ) ) {
		return;
	}

	// Get shipping total.
	$shipping_total = WC()->cart->get_shipping_total();

	// Only add fee if there's shipping cost.
	if ( $shipping_total > 0 ) {
		$tariff_amount = $shipping_total * $tariff_rate;
		
		// Add the fee (false = not taxable).
		WC()->cart->add_fee(
			__( 'US Import Tariff (Shipping)', 'woocommerce' ),
			$tariff_amount,
			false,
			''
		);
	}
}
add_action( 'woocommerce_cart_calculate_fees', 'add_us_shipping_tariff_surcharge', 10 );