WooCommerce Subscriptions – reduce the sign-up fee by a set amount when a recurring amount discount is added to the cart

wcs_sign_fee_discount_with_recurring.php

<?php

add_action( 'woocommerce_cart_calculate_fees','wcs_sign_fee_discount_with_recurring', 10, 1 );

function wcs_sign_fee_discount_with_recurring( $cart ) {
	global $product;
	
	// set 999 equal to the post ID of the subscription product
	$product_id = '999';
	$product = wc_get_product( $product_id );

        // set 2EAPV4V3 equal to the recurring coupon
        $coupon_code = '2EAPV4V3';

	// set 0.09 equal to the percentage discount; must be a positive number
	$percentage_discount *= 0.09;

        if( in_array( wc_format_coupon_code( $coupon_code ), $cart->get_applied_coupons() ) && empty( $cart->recurring_cart_key ) ) {
		// adding the title of the 'fee', in this case a sign-up fee discount
                $title = __('Sign-up fee discount', 'woocommerce');
                $cost  = -($product->get_price() * $percentage_discount);
                $cart->add_fee( $title, $cost, false );
        }
}