Maybe this solution already has been posted and I'm beating a dead horse, but I'm surprised that none of the 'experts' mentioned this to the people asking for help.
To have zencart run in an iframe is pretty simple, in order to pass payments to paypal for processing all you have to do is submit the form FROM the zencart store page. All you need for this is to submit the form to a new window.
I.e. add target=_blank in the <form> declaration attribute.
To do this:
open includes\functions\html_output.php:
line 356 change parameters declaration value (look at $parameters) from:
function zen_draw_form($name, $action, $method = 'post', $parameters='' )
to
function zen_draw_form($name, $action, $method = 'post', $parameters)
then, open
templates\tpl_checkout_confirmation_default.php (in whatever templates folder you're using classic or custom)
and on line 174 which looks like this:
echo zen_draw_form('checkout_confirmation', $form_action_url, 'post', 'id="checkout_confirmation" onsubmit="submitonce();"');
add target=_blank between id and onsubmit, so that it looks like this:
echo zen_draw_form('checkout_confirmation', $form_action_url, 'post', 'id="checkout_confirmation" target=_blank onsubmit="submitonce();"');
upload these changes to your server and voala, no need to go through the drudgery of customizing your site to fit into zen cart.