As mentioned in the Create a Shipment section of the API documentation, you are able to filter the Carriers that are returned in the response by passing in either a single CarrierAccount ID, or an array of CarrierAccount IDs. This will limit the rating calls to only the specified carriers, which will save time and bandwidth on your rating calls.
If the provided CarrierAccount IDs are invalid or disabled (you can find more information on creating/disabling carrier accounts within EasyPost here), the shipment create call will instead result in an error.
If no CarrierAccount IDs are provided on a rating call, the EasyPost API will instead perform a rate shop with all CarrierAccounts currently associated with the User/API Key that is used to make the call.