I certainly wouldn't trust it.

I use satchels for that reason. Flat rate Australia-wide and the costs are known upfront.

Generally, from eBay's own policies, offers are ex-postage, so they are for the item. Postage is whatever it normally is.

There is a fair chance, unless your items are bulky, that using flat rate satchels/boxes is likely to be the most cost effective. Certainly in the long term.

Edit: you have everything 'free' postage. You should already know what the postage is, so factor that into whether or not an offer is viable.