in the WA_eCart/WA_eCart_UPS_PHP.php file, change line 47:
$UPS_XML .= "<PostalCode>".$UPS_Parameters["ShiptoZip"]."</PostalCode><CountryCode>".$UPS_Parameters["ShiptoCountry"]."</CountryCode></Address></ShipTo><Service><Code>".$UPS_Parameters["CarrierCode"]."</Code></Service>";
to:
$UPS_XML .= "<PostalCode>".$UPS_Parameters["ShiptoZip"]."</PostalCode><CountryCode>".$UPS_Parameters["ShiptoCountry"]."</CountryCode><ResidentialAddressIndicator>1</ResidentialAddressIndicator></Address></ShipTo><Service><Code>".$UPS_Parameters["CarrierCode"]."</Code></Service>";
this will default the address type to residential.