If you are transporting any goods for sale or trade its a commercial operation.
So lets compare the medical profession.
Optometrist....travels around checks peoples eyes, prescribes new glasses. The glasses come later...so no trade.
Doctors, GP travels around, gives you a flu shot.....
Hang on carried goods which is now jabbed in your arm, and his bill is $70 consultation and $30 for the vaccine.
Vet.....preg testing cows......ArmUpACowsBum might be able to comment here, but thats no problem.
But while he is there a cow has a dose of three day.....out comes the big needle...
goods
Now I really do not want to get into a legal "dung fight", but if and I say IF common sense were to apply it would go something like this.
Private = YOU, YOUR BUSINESS, YOUR OWN GOODS, MATERIALS AND FAMILY STAFF/CONTRACTORS whether or not the pilot is you or one of your staff, and no matter whether the goods/materials are being billed to your customer as part of your work or not.
Commercial = Everything else, such as taking someone elses goods that have nothing to do with you, your work or business, and being paid for it. In other words taking regular work away from freighters.
The catch with the last one is, when young Johnny gets asked by his neighbour to rent the flying school 172 and fly a coil of copper tube out to a cattle property near Thargomindah, and there is no such thing as a regular freight run there let alone overnight service. Young Johnny does not get paid for it, it is just a favour and he loves flying out bush, and the neighbours business reimburses Johnny for his flying school hire. Is this now commercial? What is the neighbour wants to go along as well, to install the air con and takes the copper tube and his tool box with him?
Kinda mucks up the "simple rule" a bit doesn't it.