I don't have any usps ID so I can't do the full test, but for me the problem come from the the URLFetch arguments you are giving. try this code:
function xmlLoader(){
var pounds = 5;
var ounces = 2;
var userid = "1000"; //"[userid]";
var url = "http://production.shippingapis.com/ShippingAPI.dll";
var payload =
{
"API" : "RateV4",
"XML" : "<RateV4Request USERID=\"" + userid + "\"> \
<Revision/> \
<Package ID=\"1ST\"> \
<Service>PRIORITY</Service> \
<ZipOrigination>02211</ZipOrigination> \
<ZipDestination>90210</ZipDestination> \
<Pounds>" + pounds + "</Pounds> \
<Ounces>" + ounces + "</Ounces> \
<Container>RECTANGULAR</Container> \
<Size>LARGE</Size> \
<Width>15</Width> \
<Length>30</Length> \
<Height>15</Height> \
<Girth>55</Girth> \
</Package> \
</RateV4Request>"
};
var options={
method:"POST",
payload:payload
}
var response = UrlFetchApp.fetch(url, options);
Logger.log(response.getContentText());
};
have a look at the doc here