Yes. In most cases, any negotiated rates you have with your carrier will automatically appear as the rate in ShipStation.
For FedEx accounts with negotiated rates, the rates will appear only after you have created your first label in ShipStation.
For UPS accounts, you must enable negotiated rates in ShipStation first. Go to
Settings > Shipping > Carriers & Fulfillment. Click Settings and then check the Enable Negotiated Rates option and enter the requested details from your most recent invoice. Then, click Confirm.