We ship via UPS and USPS (postal). Postal is great, inexpensive and fairly fast. One thing it isn't is automatically insured or tracked. Because of this we externally insure all packages including USPS but still, we found that shipping expensive things by USPS was not a great idea. That's what UPS is for, which is a trackable service that has better systems in place for valuables. So, for orders over $200, we force domestic customers to use UPS as opposed to USPS. Here's how to set an arbitrary limit on the usps shipping module.

Make sure that you have other shipping options enabled! Don't disable USPS without any good alternatives.

Also, UPS can't ship to PO Boxes. If shipping expensive things to PO Boxes is important for your business, don't do this mod!

add these lines

define('MODULE_SHIPPING_USPS_LIMIT_ERROR', 'For packages over $200 we require shipment via UPS');
define('MODULE_SHIPPING_USPS_LIMIT', 200);

Change 200 to whatever you want the limit to be!

Find line 188

    if (is_array($uspsQuote)) {

and replace with

if (is_array($uspsQuote) && $_SESSION['cart']->total < MODULE_SHIPPING_USPS_LIMIT) {

Find line 281

elseif ($uspsQuote == -1) {
      $this->quotes = array('module' => $this->title,
                            'error' => MODULE_SHIPPING_USPS_TEXT_SERVER_ERROR . (MODULE_SHIPPING_USPS_SERVER=='test' ? MODULE_SHIPPING_USPS_TEXT_TEST_MODE_NOTICE : ''));
    } 

and replace with

elseif ($uspsQuote == -1) {
      $this->quotes = array('module' => $this->title,
                            'error' => MODULE_SHIPPING_USPS_TEXT_SERVER_ERROR . (MODULE_SHIPPING_USPS_SERVER=='test' ? MODULE_SHIPPING_USPS_TEXT_TEST_MODE_NOTICE : ''));
    } elseif ($_SESSION['cart']->total > MODULE_SHIPPING_USPS_LIMIT) {
      $this->quotes = array('module' => $this->title,
                            'error' => MODULE_SHIPPING_USPS_LIMIT_ERROR);
    }