functions.php

add_action( 'woocommerce_order_details_after_order_table', 'post_sale_cross_sell' );
function post_sale_cross_sell( $order ) {
	$cross_sells = array();
	$in_order     = array();

	foreach ( $order->get_items() as $item ) {
		$product_id  = $order->get_item( $item )->get_product_id();
		$product     = wc_get_product( $product_id );
		$cross_sells = array_merge( $product->get_cross_sell_ids(), $cross_sells );
		$in_order[]  = $product_id;
	}

	$cross_sells = array_diff( $cross_sells, $in_order );
	$cross_sells = apply_filters( 'woocommerce_cart_crosssell_ids', wp_parse_id_list( $cross_sells ) );
	?>

	<section class="related products post_sale">

		<h2><?php esc_html_e( 'You might be interested in...', 'domain' ); ?></h2>

		<p><?php esc_html_e( 'People who purchased the same products you bought usually also buy the following ones. Care to take a look?', 'woocommerce' ); ?></p>

		<?php woocommerce_product_loop_start(); ?>

			<?php foreach ( $cross_sells as $product_id ) : ?>

				<?php
				 	$post_object = get_post( $product_id );

					setup_postdata( $GLOBALS['post'] =& $post_object );

					wc_get_template_part( 'content', 'product' ); ?>

			<?php endforeach; ?>

		<?php woocommerce_product_loop_end(); ?>

	</section>
	<?php
}