Yes, you should be able to enter your own postage cost. On the sell an item screen, tap on the postage and local pick up option. If you're using a 3kg Aust Post satchel, select it from the dropdown at the top. If a box doesn't then pop up to manually enter the postage cost, scroll down to where it says "show more". Select the first show more (the second one is for international postage). Tap on the down arrow next to domestic postage type and select "flat: same cost to all buyers". Then go back to the top and tap on the dropdown arrow where you've added the satchel and you should now get a box pop up to enter the postage amount. Remember to add a bit extra to cover the postage fees.