It's an interesting aspect and I wonder the extent that small local convenience store sub-post office counters avoid some of the problems. One local sub-Post Office I tend to use these days is in a small village convenience store, just a small counter. Store is almost always empty of customers so person serving me is going to be there anyway to serve shop customers, they just wander across to the PO counter, take my parcel, etc. then when done wander back to the shop counter to wait for next customer. I don't know the economics of small convenience stores but my impression is that such outlets have no additional staffing costs and lose a tiny area of the shop that they are paying for anyway. But I can see there maybe some extra costs eg insurance with a lot more cash in the safe.
But then there's the aspect of "trust" in trusting those ("supplier side")you are doing business with ...
Ian