Looks like Shiprush has their software working with Fedex and Stamps.com now. You can find out more info at…
http://stamps.shiprush.com/cscart/cscart_index.shtml
…and…
http://fedex.shiprush.com/cscart/index.shtml
It doesn't look like they've done anything with UPS yet.
Interesting, we now have the “option” of processing shipments directly through CS-Cart, or from within Quickbooks.
We have been using ShipRush Version 9 for processing both our Fedex as well as Stamps.com shipments for about a month now & utilizing the Regional Rate A&B Priority Mail rates are saving us a small fortune. Although we process all shipments from within QuickBooks & just post the shipment details back to CS-Cart, it is nice to now have the option to process w/ShipRush thru CS-Cart if beneficial to some. 
The part that I like is when I process through Shiprush it is supposed to put in the tracking number and mark the order “complete” automatically when we ship it. You have to upload a script that talks to your CS admin and it does the work instead of us. We got it all working so far but it is not marking the orders complete or putting in the tracking number yet. I'm waiting for Shiprush support to get back with me so we can get that working too. We are trying to speed up the process and not have to copy and paste so much info. That is the big thing we are shooting for.
We also got everything in Shiprush hooked up to Ebay and Amazon so far and it is working great.