I cant see why you are getting the error, but you can manually make the change by editing the following code block:
<?php
//WA eCart Redirect Check Out
if (isset($_POST["eCart1_Checkout_1"]) || isset($_POST["eCart1_Checkout_1_x"])) {
$Redirect_redirStr="";
if ($Redirect_redirStr != "") {
if (isset($_SERVER['QUERY_STRING']) && $_SERVER['QUERY_STRING'] != "") {
if (strpos($Redirect_redirStr, "?") === false) {
$Redirect_redirStr .= "?".$_SERVER['QUERY_STRING'];
}
else {
$Redirect_redirStr .= "&".$_SERVER['QUERY_STRING'];
}
}
$eCart1->redirStr = $Redirect_redirStr;
}
$eCart1->cartAction = "Checkout";
}
?>
and setting this line:
$Redirect_redirStr="";
to point to the checkout page:
$Redirect_redirStr="https://www.mydomain.com/checkout.php";
the checkout wizard is not reentrant, you would need to delete the checkout pages and re run the wizard.