Use jQuery to move a Register Helper field on the Checkout page.

move-something.js

jQuery(document).ready(function($){
	$('#pmpro_county_div').appendTo($('#bzipcode').parent());
	$('#pmpro_checkout_box-checkout_boxes').css('display','none');
});

pmpro-move-reg-helper-field.php

<?php
/**
 * Plugin Name: PMPro Customizations 
 * Plugin URI: https://www.paidmembershipspro.com/wp/pmpro-customizations/
 * Description: Customizations for positioning Register Helper field. Add this php file and .js file to Customizations plugin's folder.
 * Version: .1
 * Author: Paid Memberships Pro
 * Author URI: https://www.paidmembershipspro.com
 */

function add_counties_field_to_checkout() {
	if ( ! function_exists( 'pmprorh_add_registration_field' ) ) {
		return;
	}

	$pmpro_counties = create_county_fields_for_profile();
	$fields = array();
	$fields[] = new PMProRH_Field(
		'pmpro_county', 'select', array(
			'label' => 'County',
			'profile' => true,
			'location'  => 'after_username',
			'required' => false,
			'options' => array_merge(
				array(
					'' => '- choose one -',
				),
				$pmpro_counties
			),
		)
	);
	// add the fields into a new checkout_boxes are of the checkout page
	foreach ( $fields as $field ) {
		pmprorh_add_registration_field( 'checkout_boxes', $field );
	}
}
add_action( 'init', 'add_counties_field_to_checkout' );


function create_county_fields_for_profile() {
	$counties = array(
		'alamance' => 'Alamance',
		'alexander' => 'Alexander',
		'alleghany' => 'Alleghany',
		'anson' => 'Anson',
		'cabarrus' => 'Cabarrus',
		'caswell' => 'Caswell',
		'catawba' => 'Catawba',
		'chatham' => 'Chatham',
		'davidson' => 'Davidson',
		'davie' => 'Davie',
		'forsyth' => 'Forsyth',
		'gaston' => 'Gaston',
		'guilford' => 'Guilford',
		'iredell' => 'Iredell',
		'lincoln' => 'Lincoln',
		'mecklenburg' => 'Mecklenburg',
		'montgomery' => 'Montgomery',
		'moore' => 'Moore',
		'randolph' => 'Randolph',
		'richmond' => 'Richmond',
		'rockingham' => 'Rockingham',
		'rowan' => 'Rowan',
		'stanly' => 'Stanly',
		'stokes' => 'Stokes',
		'surry' => 'Surry',
		'union' => 'Union',
		'wilkes' => 'Wilkes',
		'yadkin' => 'Yadkin',
	);
	return $counties;
}

add_action( 'wp_enqueue_scripts', 'add_jquery_to_move_something' );
function add_jquery_to_move_something() {
	wp_enqueue_script( 'move-something', plugins_url( 'move-something.js', __FILE__ ), array( 'jquery' ), '1.2' );
}