adds-a-fee-when-a-subscription-product-is-added-into-the-cart.php

add_action( 'woocommerce_cart_calculate_fees', function() {
	if ( wcs_cart_contains_renewal() ) {
		return;
	}
	$subscription_box_id = 54;
	$cart_contents = WC()->cart->get_cart_contents();
	foreach ( WC()->cart->get_cart() as $cart_item ) {
		if ( empty( $cart_item[ 'wcsatt_data'][ 'active_subscription_scheme' ] ) ) {
			continue;
		}
		$product = $cart_item[ 'data' ];
		if ( $subscription_box_id == $product->get_id() ) {
			WC()->cart->add_fee( sprintf( __( 'Sign Up fee for "%1$s"', 'woocommerce_all_products_for_subscriptions' ), $product->get_title() ),50 );
		}
	}
} );