The households that might choose to spend $136 per year to avoid junk mail are probably wealthier than average and therefore more valuable to advertisers. So letting those households opt-out would also probably reduce the value of those that remain. As a wild guess I suspect they might might need to charge closer to ~$200/yr to be revenue neutral to the USPS.
Having said that there are cheaper options [2] and I don't think the USPS should have to resort to delivering ~41 pounds of unwanted junk to every citizen every year just to stay afloat.
[1] https://about.usps.com/what/financials/integrated-financial-... [2] https://www.paperkarma.com/