When using an integrated payment method -- such as Paypal, Authorize.net, Moneris, et cetera -- you will need to have the payment and the postback from the payment happening on the same site address, and that site address will need to be marked as the primary domain address for that site.
This site is secure on ncopbcs.com and www.ncopbcs.com, and still has the temporary domain address of ncopbcs.chi.v6.pressero.com. The option for Redirect to Primary Domain is not enabled.
In this case a site user could come to the site on either www.ncopbcs.com or ncopbcs.com, add a product to cart, checkout and make a payment. The error would be shown because the primary domain for the site is marked as ncopbcs.chi.v6.pressero.com and that is the address the payment provider will be trying to return the postback to.
The solution is to mark one of the custom domains as the primary domain address for the site, enable Redirect to Primary Domain, and save the settings for the site.
Having Redirect to Primary Domain enabled means that a user could go to any of the site addresses in the browser, and the site will always load up on the primary domain address. Since any payments on checkout, and the postback from the integrated payment method will always happen on the same address, the error will be resolved.