E-commerce module and new forms API

Hello,
I wanted to uprgrade existing 4.6 ecommerce module to the new forms API in 4.7.
I have used formupdater.module and documentation, but not all aspect of using new API is clear to me.

Please tell me how to put custom output to the $form array i.e in the store_settings function

all I want to return is the $form array as the documentation suggest but there is still need to add additional output like:

t('<h3>Store module dependencies</h3>');

and

theme('table', $header, $row)

I tried to use

$form = array('#type' => 'markup', '#value' => theme('table', $header, $row));

but it doesn't work :(

Please give me some hints.