Sliced Invoices: in wp-admin area, show quotes and invoices only to admin users

functions.php

function sliced_custom_params_admins_only( $opts ) {
	$opts['map_meta_cap'] = false;
	$opts['capabilities'] = array(
		'edit_post'          => 'manage_options',
		'read_post'          => 'manage_options',
		'delete_post'        => 'manage_options',
		'edit_posts'         => 'manage_options',
		'edit_others_posts'  => 'manage_options',
		'delete_posts'       => 'manage_options',
		'publish_posts'      => 'manage_options',
		'read_private_posts' => 'manage_options',
	);
	return $opts;
}
add_filter( 'sliced_invoice_params', 'sliced_custom_params_admins_only' );
add_filter( 'sliced_quote_params', 'sliced_custom_params_admins_only' );