This tutorial outlines a quick and easy way of securing Ubercart by redirecting the cart and other directories to the SSL version.
Add the text below to the .htaccess file in your Drupal root directory AFTER any Drupal redirects you are already using. You will need to modify the RewriteRule lines below if your URLs do not match the ones I use in this example.
# Start code copy RunSSL.com Ubercart .htaccess redirect to SSL.
#
# Technique below is preferred over RewriteCond %{SERVER_PORT} 80
# because it does not require mod_ssl to be loaded