"A postbus is a public bus service that is operated as part of local mail delivery as a means of providing public transport in rural areas ...
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with lower levels of patronage, where a normal bus service would be uneconomic or inefficient. Postbus services are generally run by a public postal delivery company and combine the functions of public transport and mail delivery and collection. A standard passenger fare is payable to the driver or mail carrier." - (en.wikipedia.org 07.11.2021)