Running 1.38a+security+FEC, with usps shipping module enabled for priority and 'first class'. When I run validator.w3.org on the page, it reports the following (and other related) errors:

Code:
Line 276, Column 126: value of attribute "id" must be a single token

…="updateForm();" id="ship-usps-FIRST CLASS" />	  <label for="ship-usps-FIRST 

This attribute can not take a space-separated list of words as a value, but only one word ("token"). This may also be caused by the use of a space for the value of an attribute which does not permit it
I've looked via the Developer's ToolKit and 'first class' is a database entry for the usps module and it's the space between 'first' and 'class' that gets the validator's knickers in a bunch.