Use flat rate up to a certain weight, then use USPS

[kobra]

Disabling flat rate will still work
Might take a bit of code to not show USPS until a certain weight is acheived

[Ajeh]

Just use something similar in the usps.php ...

    // disable only when entire cart is free shipping
    if (zen_get_shipping_enabled($this->code)) {
      $this->enabled = ((MODULE_SHIPPING_USPS_STATUS == 'True') ? true : false);
    }

// bof: If the shipping weight is less than 20lb disable the module
if (!IS_ADMIN_FLAG) {
  global $cart;
  if ($_SESSION['cart']->show_weight() < 20) {
    $this->enabled = false;
  }
}
// eof: If the shipping weight is less than 20lb disable the module