This was found in troubleshooting today. OPC installed in Zen Cart 1.5.8 and then added the new Square WebPay version 1.0.3. We were getting stopped at Confirm Checkout with a token error.

For those wondering about 1.5.8, WebPay works exactly as expected in Zen Cart 1.5.8 WITHOUT One Page Checkout.

If you are going to use both One Page Checkout AND Square WebPay with Zen Cart, you will need to get the OLD square javascript file from includes/modules/pages/checkout_payment/ called jscript_square.php.

Copy or FTP that file to includes/modules/pages/checkout_one/.

The includes/modules/pages/checkout_payment/jscript_square.php is still part of the 1.5.8 distribution zip file. It may not be part of further distros.