I was using Chrome when I tested and didn't had any redirect issue. There was no iframe on your website so no need to display that error message.
I'm guessing that maybe you're accessing internet via a proxy (maybe your internet provider ?) and it adds an iframe around the website or something ?
Since everything is ok for me on your website, there is nothing to fix as far as I can see. However, if the error message you have says that the problem comes from an iframe issue, the solution should be easy.
Edit the file plugins/hikashoppayment/paypal/paypal_end.php and change the line:
<form id="hikashop_paypal_form" name="hikashop_paypal_form" action="<?php echo $method->payment_params->url;?>" method="post">
to:
<form id="hikashop_paypal_form" name="hikashop_paypal_form" action="<?php echo $method->payment_params->url;?>" method="post" target="_blank">
That way PayPal payment gateway will open on a new page and you should not have iframe issues.