Is this just the code from the snippet directly or did you generate this code with the checkout wizard or by applying the individual server behavior?
When I choose the First Class mail type the code that eCart produces for me does not have the FirstClassMailType. You should try removing this from your code to see if you get past this error:
$Shipping_Parameters["FirstClassMailType"] = "PARCEL";
If your current version of eCart is producing this code please let me know.