I have UPS Simple Rates working on ShipStation. UPS tech support and my account manager were very helpful getting this set up - I previously opened a ticket with ShipStation but they didn't know how to do it. You'll need to contact your UPS account manager and create a separate UPS account for Simple Rates (I'm told that the Simple Rates account isn't able to get the discounts for shipping non-simple rate shipments). Once you have the second account configure it in ShipStation as a second UPS account and be sure to enable Negotiated Rates. Leave your original UPS account as the primary UPS shipping account. Now when you want to ship a package with Simple Rate open the order in ShipStation and under "Other Shipping Options" select the UPS Simple Rate Account - you'll then see the simple rate prices for shipping that package.
I would really like to see this added. For some of our packages, this program makes a lot of sense, and I really do not want to have to start shipping packages through multiple software solutions.