Getting error "courier_selection.selected_courier

Following on from this question https://developers.easyship.com/v1.0/discuss/647e9ef82a7075003bba3f3a.

I have successfully found a courier_id for Australia Post by integrating Australia Post with my account and using my production API key to list all couriers at /couriers. However, when I try and use the listed id in /shipments, it says

  "courier_selection.selected_courier_id is not a valid courier id"

The id I am trying to use is:

571ffba3-59f9-4a14-b497-600f523f1bf2

Which is for the following option:

Pop MyPost - MPB - Parcel Post Flat Rate Box Small
ParcelPostFlatRateBox

I am wondering how to proceed.

Thanks,

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Getting error "courier_selection.selected_courier_id is not a valid courier id"