You could force Standard Post at >= 6, but then no other shipping methods will work at >= 6 ...
Before attempting this, check that Standard Post will be available to you on orders near and far ...
You could customize the USPS shipping module:
/includes/modules/shipping/usps.php
with the code in RED:
Code:
// when $usps_groundonly is set to true, only Stand Post will show
$usps_groundonly = 'false';
// bof: force Standard Post Only >= 6
if ($shipping_weight >= 6) {
$usps_groundonly = 'true';
}
// eof: force Standard Post Only >= 6
// $usps_groundonly = ($_SESSION['cart']->in_cart_check('products_groundonly','1') ? 'true' : 'false');
if ($usps_groundonly == 'false') {