I found that the body field in Donation products doesn't support editors implemented via the WYSIWYG module, at least in Drupal 5. I was able to remedy this by making the following change to line 104 of uc_donation.module:

   $form['body_filter']['filter'] = filter_form($node->format);

becomes:

   $form['body_filter']['format'] = filter_form($node->format);

Hope this will be of use to others.