Hello,
The built-in UPS shipping mod did not support Zip+4 postcodes. I checked out the USPS mod and they had a simple fix for it, so I ported it over to the UPS mod. This seemed like a no-brainer to me and it makes me wonder why this wasn't there in the first place.
/modules/shipping/UPS/calc.php
Lines 42 if (preg_match('#product#', $key) && $value) {
$productCode = substr($key,7,3);
// fix for US zipcodes e.g. 12345-1234