How much more useful would the USPS be if:

- Taxpayer support for the service were increased.

- Junk mail delivery were reduced.

?

Perhaps even more importantly, what would the environmental impact be?

Interesting question.

I'm not sure what total junk-mail volume is, but the USPS moved 127.3 billion "units" (pieces of mail) in 2022, down from a peak of 213 billion in 2006.

First-Class mail volume was 48.9 billion in 2022, single-piece (which I presume means non-bulk) 12.9 billion.

"Marketing mail" volume, 67.1 billion.

<https://facts.usps.com/table-facts/>

First-class letter max weight is 3.5 oz (100 g).

Assuming an average weight of 100g per item, that's 6.7 million tonnes of marketing (junk) mail, give or take an order of magnitude.

A tractor-trailer rig can carry a maximum of about 45,000 lb (20 tonnes).

The junk-mail delivered in a year is roughly 400,000 such truckloads of mail. Or assuming 5-day/week delivery, about 2,000 trucks operating daily for a year.

(I'm using round numbers as the initial estimate is rough, just trying to give rough scope to the scale.)