Other articles* mention fees: $5.95 to cash checks up to $500. That's more than my local-chain grocery store's service counter.
* https://prospect.org/economy/usps-begins-postal-banking-pilo...
* https://prospect.org/economy/usps-begins-postal-banking-pilo...
It would be great in principle if the USPS could cash checks for free, but they are also taking on the risk of the check being bad.