It's extremely disappointing that USPS has gone the way of the Ordnance Survey on this. UK postcode polygons (at the unit level) have been paywalled for some time, but its even worse in that as far as I know there's no way to even get a straight answer as to how much it will cost to get the Ordnance Survey postcode products' data.

It's especially annoying in the UK case because there is such rich national data, but for so much of it the only join key is the postcode unit, which can change over time and for which no public polygon is available. Evidently the Ordnance Survey doesn't even keep historic vintages of its releases...