Symptom:
Normally during checkout you want it to auto-select the cheapest shipping option for your customer.
But in some configurations this may not happen, and thus no radio-buttons are pre-selected.
This affects versions v1.5.3 and v1.5.4.

Cause:
There was a condition missed when rewriting that section of the code for PHP 5.4 compatibility.

Fix:
The solution is simple:

/includes/modules/pages/checkout_shipping/header_php.php, line 212:

Change this:
Code:
  if ( !isset($_SESSION['shipping']) && (zen_count_shipping_modules() > 1) )  $_SESSION['shipping'] = $shipping_modules->cheapest();
to this:
Code:
    if ((!isset($_SESSION['shipping']) || (!isset($_SESSION['shipping']['id']) || $_SESSION['shipping']['id'] == '') && zen_count_shipping_modules() >= 1)) $_SESSION['shipping'] = $shipping_modules->cheapest();