You could customize the usps.php shipping module to not show Priority when the weight is .5 by editing:
/includes/modules/shipping/usps.php
and adding the code in RED:
Code:
// add $this->usps_countries to title to test actual country
// bof: skip PRIORITY when weight is = .5
if ($usps_shipping_weight == .5 && $type == 'PRIORITY') {
// skip shipping for PRIORITY on weight = .5
} else {
$methods[] = array('id' => $type,
'title' => $title,
'cost' => ($cost * $shipping_num_boxes) + (MODULE_SHIPPING_USPS_HANDLING_METHOD == 'Box' ? $usps_handling_fee * $shipping_num_boxes : $usps_handling_fee) );
}
// eof: skip PRIORITY when weight is = .5
// bof: sort by contributed by Marco B