Here is the code that changes the default select list of "Select a product" element of Drupal Commerce to radio buttons, for a standalone product list.
The following goes in a custom module (tested in Drupal 7).
<?php
function MYMODULE_form_alter(&$form, $form_state, $form_id) {
if (strpos($form_id, 'commerce_cart_add_to_cart_form_') === 0 && isset($form['product_id'])) {
$form['product_id']['#type'] = "radios";
}
}
?>