And in no instances have I ever run across use cases where these are worth paying any substantial money for. If USPS is going to jump into the fray here, really bad look (and probably net negative revenue for them) to offer as a paid product.
Whatever you're grouping is likely better served by some grouping based on geography, not based on delivery routes. You probably wouldn't group by telephone company central office either, unless you're telco related.
We want to group by geography, zip codes are easy and vaguely geographic, lets use that is kind of a valid decision, but it's deeply frustrating :p
In fact, these were invented by Nielsen, and they determined them just by measuring which city's TV stations are predominantly watched in each county, and assigning each county to one DMA based on that.