You can use in_array to check if the name of the role is either customer or shop_manager
add_action('register_form', 'myplugin_register_form');
function myplugin_register_form() {
global $wp_roles;
echo '<select name="role" class="input">';
foreach ($wp_roles->roles as $key => $value):
if ( in_array( $key, array('customer', 'shop_manager') ) )
echo '<option value="' . $key . '">' . $value['name'] . '</option>';
endforeach;
echo '</select>';
}