Before being able to select the shipping service and carrier, you need to complete Step 1: Parcel details section, where you have postal codes of origin and destination and the size/measurements of your parcel.
Postal codes will be automatically imported from PayPal, please double check that they are correct.
You now have to indicate Weight, Length, Width and Height. If you have not configured your default parcel settings, you will have to enter the weight and measurement of your parcel at this point.
If you don't want to add the measurements of your parcel each time, you can set up default parcel which will save you time in your next shipment: Default parcel size
If you have already added your parcel details, you can now select the carrier and service. You can access the service selection by visiting the Step 2: Service selection.
If you have already selected the shipping service in your PayPal listing, it will be automatically imported in your PayPal Shipping account, otherwise, the first service available will be assigned to your shipment by default.
You can always change the carrier and service from the table view, click on "edit" and change it to something of your choice.
You can also click on "book" and then go back to Service Selection to check the full list of services available.