AFAIK the PO gets a cut of each parcel postage price (I could be wrong, but that’s the impression I get from mine) although I’m not sure if the Postmaster is salaried or not. He did tell me that they didn’t make anything on parcels paid for on the internet though.
But your guy sounds like he’s an utter arse, to be honest. Lots of POs are like this. Farnborough Main PO is staffed with jobsworths. It’s a big place, yet the small PO in Tongham near where I work will bend over backwards to help. I mentioned poting a bike the other day and that I’ve always gone straight to the Parcelforce depot, but he said If I can get it in the door, he’ll accept it!