Clicking checkout in the cart auto creates an order. Must have Cash On Delivery gateway enabled.

wc-auto-checkout.php

<?php
/*
 * Plugin Name: Auto Checkout for WooCommerce
 * Plugin URI: https://www.paypal.me/helgatheviking
 * Description: Clicking checkout in the cart auto creates an order. Must have Cash On Delivery gateway enabled.
 * Version: 1.0.0
 * Author: Kathy Darling
 * Author URI: http://kathyisawesome.com
 * Requires at least: 5.3.0
 * WC requires at least: 3.8.0    
 * Tested up to: 5.3.0
 * WC tested up to: 3.8.0   
 *
 * Copyright: © 2019 Kathy Darling.
 * License: GNU General Public License v3.0
 * License URI: http://www.gnu.org/licenses/gpl-3.0.html
 *
 */


/**
 * Add a query arg to checkout url.
 */
function kia_auto_checkout( $url ) {
	return add_query_arg( 
		array( 
			'auto-checkout'                      => 'true',
			'woocommerce-process-checkout-nonce' => wp_create_nonce( 'woocommerce-process_checkout' )
			),
			$url
		);
}
add_filter( 'woocommerce_get_checkout_url', 'kia_auto_checkout' );

/*
 * Remove all checkout fields.
 */
function kia_remove_fields_for_auto_checkout() {
	add_filter( 'woocommerce_checkout_fields', '__return_empty_array' );
	add_filter( 'woocommerce_default_address_fields', '__return_empty_array' );
	add_filter( 'woocommerce_billing_fields', '__return_empty_array' );
	add_filter( 'woocommerce_cart_needs_shipping_address', '__return_false' );
	add_filter( 'woocommerce_enable_order_notes_field', '__return_false' );
}
add_action( 'woocommerce_init', 'kia_remove_fields_for_auto_checkout' );

/**
 * Process the checkout link.
 */
function kia_auto_checkout_action() {
	if ( isset( $_GET['auto-checkout'] ) ) { // phpcs:ignore WordPress.Security.NonceVerification.NoNonceVerification
		wc_nocache_headers();

		if ( WC()->cart->is_empty() ) {
			wp_safe_redirect( wc_get_cart_url() );
			exit;
		}

		wc_maybe_define_constant( 'WOOCOMMERCE_CHECKOUT', true );

		WC()->checkout()->process_checkout();
	}
}
add_action( 'wp_loaded', 'kia_auto_checkout_action', 20 );

/**
 * Manually defind some missing checkout form data.
 * I think payment_method may be the only one you actually need.
 */
function kia_auto_set_checkout_data( $data  ) {
	return array(
		'terms'                              => 1,
		'createaccount'                      => 0,
		'payment_method'                     => 'cod',
		'shipping_method'                    => '',
		'ship_to_different_address'          => 0,
		'woocommerce_checkout_update_totals' => 0,
	);
}
add_filter( 'woocommerce_checkout_posted_data', 'kia_auto_set_checkout_data' );

/**
 * Remove the auto-checkout so we don't get redirected back to an empty cart.
 */
function kia_remove_auto_checkout_query_args( $url ) {

	return remove_query_arg( 
		array(
			'auto-checkout',
			'woocommerce-process-checkout-nonce'
		),
		$url
	);
}
add_filter( 'woocommerce_get_return_url', 'kia_remove_auto_checkout_query_args' );