In ShipStation, it's really easy to connect your Stamps.com, FedEx, UPS, and other providers to your ShipStation account.
Head to your Account Settings (the gear icon in the upper right).
Next, select Shipping and then Carriers & Fulfillment from the sidebar on the left.
Once you do this, you'll need to click on Add a Provider Account.
You'll find appropriate accounts (USPS by Stamps.com, FedEx, UPS, DHL, etc.) and follow the on-screen instructions to add a provider.
Once you've done this for each of your accounts, you should be able to get your discounted and negotiated rates within the app!