This is a very US-centric take. In the UK, EU and Australia, interchange rates are capped at more like 0.2/0.3%.
In the U.S., many credit cards bundle short term credit, rewards, travel benefits, insurance etc. This bundling is why merchants can pay up to 2-3% in fees.
The payment clearance and settlement parts are much cheaper. That's why many countries are able to build domestic payment rails (e.g. Pix in Brazil) that process transactions at low cost.
Which I'm hoping will find its way back to our wallets but which I doubt will happen
Government doesn't always mean worse.
Certain individuals, such as Susan Collins and Tom Davis, who were the largest names behind the 2006 law to force this on the USPS, were clearly doing it in an attempt to make the USPS fail/become less competitive. PACs affiliated with FedEx, UPS, etc gave more than $192,000 to Collins’s campaign / PAC from 2000 to 2004. FedEx’s PAC has given Collins more donations than any other congressional candidate in that PAC's history.
There is a huge chain of evidence suggesting members of congress don't want the USPS to succeed due to self interest.
Tangent, having lived in 5 states, the USPS workers have always been nice and courteous, even when there are lines out the door during the tax season. And in general, like DMV or other licensing agencies.