The five ISC centers (created in 1996) are massive clearinghouse's only for goods in or out of the country. It was kind of an experiment in 'int'l procssing efficiency'. The incoming stuff all gets dumped into their processing bins, and then as time permits, clearances are checked and then these are dispatched to traditional regional distribution centers. I think it's fair to say the ISC is a huge bottleneck, and that waiting for inbound to become outbound through an ISC could easily take 2 weeks. The ISC sounded good on paper, but really hasn't benefitted anyone...

GoShippo.com offers a "Parcel Select" rate through USPS which is NOT available to retail customers otherwise! That is how the save money, by offering the special PS rate as an aggregator for single packages: From the USPS bulletin site:
“Parcel Select” is the registered trademark name for the Postal Service's economical ground delivery service for packages entered in bulk, including those entered at destination facilities. It is designed for and generally used by large- and medium-sized parcel shippers.

I have used it a number of times in the recent months and find it saves 30-40% or more over the rates you'll get from the USPS.COM website directly. I had a medium box, weighing 5-6 pounds going CA to AZ recently, USPS.com quoted $27 and goshippo quoted $14. GoSHippo takes 2 minutes to sign up for and is pretty easy to use. It generates a regular USPS label just like you would expect!