After the consumer places the order, we send them back to whatever
page they last viewed in your site (or some other page you specify).
The order form is secure, and
the regular site pages aren't.
Whenever you redirect from a secure
to an insecure page, the browser issues a warning. And unfortunately,
all the security warnings issued by the browser sound equally terrifying,
whether they are really important or (as in this case) not.
If consumers worry about this message, you can safely reassure them
that their credit card info was not exposed. The message is merely
about the page they get sent to after ordering.