In Australia it is pretty easy to get a feel for what low volume retailers are paying, eBay sellers will usually charge about $7 for an auspost parcel under 500g. A little above the rate for sending to the same capital city or other major cities, below the rate of seinding the most other places.

So a retailer with a bulk discount can probably get away with that rate, much higher than around $8 though and people would know it isn't a real rate.