Nah, they're not trying to strangle the USPS. They're trying to fatten it up for privatization.

If you fully fund the pensions before privatization, you can simultaneously claim that privatization is necessary (look! they're running out of money!) and radically reduce costs once you do privatize (all the pensions are pre-paid, so you can stop contributing to the fund and extract profits for a few decades before you have problems).

This zip code subscription is part of the same plan: Get people used to paying for data from the USPS so it has an additional income stream after privatization.