Sliced Invoices: automatically send invoice to client upon invoice creation

functions.php

add_action( 'new_to_publish', 'sliced_auto_send_invoice', 10, 1 );
add_action( 'draft_to_publish', 'sliced_auto_send_invoice', 10, 1 );
add_action( 'pending_to_publish', 'sliced_auto_send_invoice', 10, 1 );
add_action( 'sliced_auto_send_invoice_deferred', 'sliced_auto_send_invoice_deferred_function', 10, 1 );
function sliced_auto_send_invoice( $post ) {
	if ( $post->post_type === 'sliced_invoice' ) {
		// run in background
		wp_schedule_single_event( time(), 'sliced_auto_send_invoice_deferred', array( $post ) );
	}
}
function sliced_auto_send_invoice_deferred_function( $post ) {
	if ( $post->post_type === 'sliced_invoice' ) {
		$sliced_notifications = new Sliced_Notifications();
		$sliced_notifications->send_the_invoice( $post->ID );
	}
}