Checking address fields for presence of a Post Office Box
There's 2 obvious places to clean this up a bit.  The first is the duplicated keyup handler and the second is the check for PO.

Most jQuery functions can be called on a collection of elements.  Since you are calling the same code on both your shipping address and billing address, we can combine them like so:

var $addresses = $('#checkout input[name="user_data[s_address]"]')
                 .add($('#checkout input[name="user_data[b_address]"]));


For the PO check, you can just use a regular expression:

//p, optional period, o
if(/p\.?o/i.test($value)) { ... }


So the whole thing becomes:

(function ($) {
    'use strict';

    // Check for PO Box text in address fields on checkout
    var $addresses = $('#checkout input[name="user_data[s_address]"]')
        .add($('#checkout input[name="user_data[b_address]"]')),
        message = "&lt;div class='message hidden'&gt;&lt;p&gt;We &lt;strong&gt;don't ship to PO Boxes&lt;/strong&gt;. Sorry for the inconvenience.&lt;/p&gt;&lt;/div&gt;";

    $addresses.after(message);

    $addresses.keyup(function () {
        var $value = $(this).val();

        if(/p\.?o/i.test($value)) { 
            $(this).siblings('.message').removeClass('hidden');
        } else if ($value === '') {
            $(this).siblings('.message').addClass('hidden');
        }
    });

}(jQuery));


See: http://jsfiddle.net/6RnxG/7/