Use Customer Email as Reply-To Email

We'd like to set the customer's email as the reply to email so our clients can quickly reply to the customer.
How can we do this?

In the config under the E-mail tab you can configure the reply to address for all the HikaShop emails.

Then, in the Business edition, you can configure a reply to address for each email via the menu System->Emails.

Is there a variable or a string that if we enter into "Reply-to Address" on the "Sender Information" in System > Email?

I don't understand your question. Could you explain more what you want ?

I am after the same option. I used this with my Virtuemart site all the time.

When i get an admin notification to say that a new order has been created, it comes from my own email address to my own email address.

what i would like as the person above suggested was a way to be able to set a string to automatically have the admin notification sent to my email address from their email address. That way the webshop administrator if they want to email them can simply reply to this email and it conveniently then includes the notification email below it as well.

You should try to add the line:
$mail->mailer->addReplyTo(array($user_email,$user_name));

after the line:
$this->loadOrderNotification($order,'order_admin_notification');

in the file administrator/components/com_hikashop/classes/order.php

I tried to implement that change, Nicolas, but got this error:
Call to a member function addReplyTo() on a non-object in .../administrator/components/com_hikashop/classes/order.php on line 150

Jim