I notice the dimension fields for parcels get greyed out when you select "Satchels and other items" - so you obviously aren't meant to even try for the cubed cost. In the previous calculator, it was always available - so that left the area of uncertainty in favour of cubing.
I can see that becoming a point of contention ... people would be reasonably ticked off if they honestly felt their parcel wasn't a 'boxed' item and rocked up at the Post Office only to get it cubed on them. As it is, there is still a grey area between when an item is 'square enough' to be cubed or not.
Obviously designed for the casual user. Haven't heard a squawk from my seller about it - but they do enough to know most of the costs just by weighing their parcels.
... Just had a quick conversation with them ... They haven't seen it yet, but when I described it, they said it sounded a lot like the USPS process - and suggested it was in preparation to online postal services.
Let the games begin!