It would be extremely helpful to have the Ship To Zip available as a built in Object, like the carrier/service/manifestPartnerId/etc., to be able to search on a customer's zip code to match to their destination zone to further enhance the Rate Shop script feature.
It looks like you should be able to do what you're looking for, by using the infoplusApi object to lookup the order you're working with (which will be identified by the orderNo variable available to the script). I believe a couple of lines of code like this would give you what you're looking for:
var order = infoplusApi.getById("order", orderNo);
var shipToZip = order.shipToZip;
Let us know if that works, or if you have any further issues.