If you have connected multiple USPS postage providers to your ShipStation account, like Stamps.com and Endicia, for example, ShipStation has the ability to optimize your rate selection. This is a feature that works with standard USPS rates.
Among these standard rates, we include Stamps.com's discounted ShipStation Pro Plan. However, if you have specially-negotiated rates such as deeper cubic discounts, these rates are not detected by the rate selector, so we advise against you using this feature.
How this works...
ShipStation displays an extra column on your cost summary screen. This column will show you the label cost and account balance details for the lower cost postage provider.
You can decide to use the optimized/recommended rates on a case-by-case basis by selecting the radio button above the extra column. But if you always want ShipStation to select optimized/recommended rates for your USPS shipments automatically, you can check the Always optimize my USPS rate selection box on the rate confirmation screen.
With that option enabled, ShipStation will default to the USPS provider with the most preferred rates.
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.
Then, look for the USPS Rate Selection section, and click Manage USPS Rate selection options.
Here, you can simply check or uncheck the box for Always optimize my USPS rate selection as needed, and Confirm.
Note: if you do not see the option to Manage USPS Rate selection options, that means you do not have multiple USPS postage provider accounts connected to your ShipStation account.