woocommerce_add_cart_item_data.php

function add_cart_item_data( $cart_item_data, $product_id, $variation_id ) {
	// Has our option been selected?
	if( ! empty( $_POST['extended_warranty'] ) ) {
		$product = wc_get_product( $product_id );
		$price = $product->get_price();
		// Store the overall price for the product, including the cost of the warranty
		$cart_item_data['warranty_price'] = $price + 250;
	}
	return $cart_item_data;
}
add_filter( 'woocommerce_add_cart_item_data', 'add_cart_item_data', 10, 3 );