PayPal WPS-IPN: How do I override the return URL?

Hi,

I'm setting up a store that uses Website Payments Std IPN as the sole payment option. We're just to the point of live testing and everything is working great except one thing. I need to be able to override the return URL (and the cancel URL for that matter) to something other than the default:

/zencart/index.php?main_page=checkout_process&referer=paypal

This Zen Cart install is running inside a frame as part of a larger site. The outer page provides the header, navigation, etc, and Zen Cart runs in an iframe in the page content area, using a basic template to reduce clutter.

I've managed to make the site break out of it's frame when it hands off to PayPal as it should be but I need it to jump back into the frames when the user comes back from PayPal. As is, Zen Cart is sending the address above as the return URL which means no frame and a confused layout. I need the return URL to be something like:

/content/shop-return.php

Which sets up the frames again and displays the Zen Cart order confirmation within it. The PayPal account is set to return to my custom return page but it's being overridden by the form data Zen Cart is posting (the 'return' and 'cancel_return' fields). I've scoured the Admin control panel but I'm not seeing it. How do I change the return URL that Zen Cart sends to PayPal to something of my choosing?
Thanks :)

Zencart version 1.3.9d (update planned before go-live).
New clean install, no addons, etc except a template.

Did you test the more advanced Express Checkout instead of the older WPS?

Thanks,
No we didn't test Express Checkout. I'll ask if switching is a possibility, though. My understanding is that we're working with WPS mainly because it doesn't require a customer to have a PayPal account (which I believe Express Checkout does require).

Wouldn't Express Checkout suffer the same problem? My understanding of the EC flow is that the customer hits the express checkout button, authorises the transaction and then returns to the store to confirm and complete. Surely at the point of return to Zen Cart, we're going to have the same problem - returning to an internal ZC page, outside the container frame?

JeremyR:

My understanding is that we're working with WPS mainly because it doesn't require a customer to have a PayPal account (which I believe Express Checkout does require).
No, it doesn't require.

JeremyR:

This Zen Cart install is running inside a frame as part of a larger site.
We've always strongly advised against doing that.