I don‘t think the leather jacket will fit in 3kg satchel. The best way to send it will be in Prepaid Parcel Post Plus5kg satchels ($15.90) with registered post ($3.20) if it fits in 5kg satchel (510mm x 435mm).

Registered post includes $100 cover. You can put extra covers for $1.50 per $100.

It will be just under $25 with extra $400 cover.