Adds the customer’s username to the "Customer Details" section on the payment screen, and links it through to the user’s profile
<?php
/*
 * Plugin Name: Easy Digital Downloads - Add Username To Customer Details
 * Description: Adds the customer’s username to the "Customer Details" section on the payment screen, and links it through to the user’s profile
 * Author: Andrew Munro
 * Author URI: https://easydigitaldownloads.com
 * Version: 1.0
 */

function sumobi_edd_customer_details_username( $payment_id ) {
	$user_info   = edd_get_payment_meta_user_info( $payment_id );
	$customer_id = $user_info['id'];

	if ( ! $customer_id ) {
		return;
	}
	
	$user_data   = get_userdata( $customer_id );
	$user_name   = $user_data->user_login;
	?>
	<div class="column-container" style="margin-top: 20px;">
		<div class="column">
			<strong><?php _e( 'Username:', 'edd' ); ?></strong><br />
			<a href="<?php echo get_edit_user_link( $customer_id ); ?>"><?php echo $user_name; ?></a>
		</div>
	</div>
	<?php
}
add_action( 'edd_payment_view_details', 'sumobi_edd_customer_details_username' );
To implement this snippet on your site, create a brand new, empty WordPress plugin and paste the above code inside.