Farming isn't all beautiful waves of grain and rolling meadows with horses frolicking. Some things need to be out in the middle of nowhere. But those places also need to be connected to the rest of the country.
Since a pig farm or a slaughterhouse needs to be in the middle of nowhere, why not make the pig farm or slaughter house pay the increased cost of providing mail delivery? Since nearly all pig farms and slaughterhouses need to be in the middle of nowhere, no one pig farm or slaughterhouse is put at a disadvantage by paying higher postage. The higher costs are simply passed on to the consumer in the form of higher prices instead of higher postage.
Note that I'm not suggesting that the USPS stop providing mail service to isolated areas (which would almost certainly be the effect of removing the USPS's limited monopoly), but charging most customers in rural areas the true cost of providing mail delivery. To the extent that some customers might not be able to afford the true cost of their mail delivery, it might be better to subsidize their relocation to other areas rather than continuing to subsidize services.