I'm not sure about letters, my PO assumed it was only on parcels the $1.50 per $100.  My PO put the info in their computer about the satchel and it came up that I was able to buy the insurance only.   PO also said that from 14 Nov we can use our own packaging instead of the satchels so we can send 3kgs in any packaging for the $11.70 printing out the label off ebay.