<?xml version="1.0" encoding="UTF-8"?>
<sf:Resource xmlns:sf="http://www.canadapost.ca/smartflow" contentID="PAYMENT">
<![CDATA[
        <payment-data>
           <account-number>bar</account-number>
            <total-due>bar</total-due>
            <due-date>bar</due-date>
        </payment-data>
]]></sf:Resource>

The following query makes use of a processor specific XQuery option called ddtek:serialize 

Notice the syntax to define an expanded QName with namespace is { {uri} local-name }  which is different from what is erroneously described in the XQuery Tips & Tricks 

The second takeaway is how the content inside sf:Resource needs to be escaped text to produce the required result

declare namespace sf="http://www.canadapost.ca/smartflow";
declare option ddtek:serialize 
 "omit-xml-declaration=no,encoding=UTF-8,indent=yes,cdata-section-elements={{http://www.canadapost.ca/smartflow}Resource}";
declare variable $foo := "bar";
<sf:Resource contentID="PAYMENT">
	    &lt;payment-data&gt;
	        &lt;account-number&gt;{ $foo }&lt;/account-number&gt;
	        &lt;total-due&gt;{ $foo }&lt;/total-due&gt;
	        &lt;due-date&gt;{ $foo }&lt;/due-date&gt;
	    &lt;/payment-data&gt;
</sf:Resource>

Multiple CDATA sections are declared using ; as separator

cdata-section-elements="{{http://www.canadapost.ca/smartflow}Resource};{{http://www.canadapost.ca/smartflow}Foo}"