shahmink - the seller is correct in saying there is a particular size for businesses only.   This satchel is bigger and costs more than what shows as being available on the AP website for the general public.   In saying that though, the sets would have weighed well over 3kg so the seller is lucky they weren't billed for underpaid postage   going by aus post a 5kg express  satchel cost $23.60 and the seller is sending in a 3kg express satchel for $14.80.....not fair on the buyer

Perhaps you should read everything. The jumbo satchel is more expensive than the standard.

If a buyer agrees to a price including postage, then the buyer has agreed to a price including postage and has no reason to consider they have been ripped off. After all, a reasonable person would assume they were happy with that total price.