The simplest thing is to default the namespace:

<order xmlns="urn:publicid:IDN+cde.berkeley.edu:example:order:en">
  <from>me</from>
  <to>you</to>
  <items>
    <item>A clue</item>
    <item>A better clue!</item>
  </items>
  <memo>
    <p>I need this order now!</p>
  </memo>
</order>