03-add-custom-user-input-woocommerce-product.php

<?php
/**
 * Add line item for monthly ad budget set by customer.
 * Added as $0 - will be adjusted in subscription later based on actual spend.
 *
 * @param obj $item           Passed by reference.
 * @param str $cart_item_key  Cart key.
 * @param arr $values         Cart values.
 * @param obj $order          Order object.
 */
public function add_order_ad_budget_item( $item, $cart_item_key, $values, $order ) {

	// If value is set then create new fee item object and add item.
	if( isset( $values['sem-ad-budget'] ) ) {

		// Add value as meta data to product in order prefixed with dollar sign.
		$item->add_meta_data('sem_ad_budget', '$' . $values['sem-ad-budget']);

		// Create new Fee object.
		$fee_item = new WC_Order_Item_Fee();

		// Set item props. Set total to 0 for later adjustment.
		$fee_item->set_props( array(
            'name'      => 'Monthly Ad Budget - $' . $values['sem-ad-budget'],
            'tax_class' => 0,
            'total'     => 0,
            'total_tax' => 0,
            'taxes'     => array(
                'total' => 0,
            ),
        ) );

		// Add the fee as a line item.
		$order->add_item( $fee_item );
	}
}