[Simple Sales Tax] Move tax exempt form after billing details

functions.php

<?php

/**
 * Moves the SST Tax Exempt form to after the customer billing details.
 * Place in your child theme's functions.php file or in a plugin.
 */
function sst_move_tax_exempt_form() {
	global $wp_filter;

	if ( ! isset( $wp_filter['woocommerce_checkout_after_customer_details'] ) ) {
		return;
	}

	$filters = $wp_filter['woocommerce_checkout_after_customer_details'];

	foreach ( $filters as $priority => $callbacks ) {
		foreach ( $callbacks as $callback ) {
			if ( ! is_array( $callback['function'] ) ) {
				continue;
			}

			list( $instance, $method ) = $callback['function'];

			if (
				get_class( $instance ) === 'SST_Checkout' &&
				$method === 'output_exemption_form'
			) {
				remove_action(
					'woocommerce_checkout_after_customer_details',
					$callback['function'],
					$priority
				);
				add_action(
					'woocommerce_after_checkout_billing_form',
					$callback['function']
				);
				break;
			}
		}
	}
}

add_action( 'init', 'sst_move_tax_exempt_form' );