I've sent over 3,000 items as large letters in Australia Post B4 envelopes and haven't got any items damaged.

However I put my items in sealed plastic bags and put sticky tapes on glued parts of the envelopes. And I also make sure my envelopes can go through Australia Post large letter gauge easily. If it doesn't go through easily it will be sent as a parcel. Fortunately most of my items are so tiny (baby clothes) and go as large letters.

After I checked the listing of Kaftan dress I'm wondering how the seller folded 94cm long long sleeve dress to fit in B4 envelopes and keep under 2cm thick with a cardboard. I don't think I can. And also I don't think the envelope with dress and a cardboard won't be under 125g. Even if it's under 2cm thickness it should cost $1.80 not $1.20.