How to pass theme variables in #theme in a form element?
Minor syntax change required...

function storeinv_uc_product_description($product)      {
  $description = array(
    'pickup' =&gt; array(
      'product' =&gt; array('#type' =&gt; 'value', '#value' =&gt; $product),
      '#theme'  =&gt;'storeinv_pickup',
      '#weight' =&gt; 1,
    ),
  );
  return $description;
}


Your product variable doesn't need to be prefixed with a hash character. Thats used for specific keys/properties of the render array.