Electronic payments completely exclude the unbanked and cost proportionally far more for low-income brackets. Cash is the only near-zero or zero cost transaction method available.
Fixing that would be a prerequisite. Public/postal banking is one way to accomplish that. Regulation is another.
I actually disagree that cash is currently near-zero cost, though. The current situation is that credit cards charge large interchange fees, and their agreements with most merchants prohibit them from passing those fees to their customer. The result is that most of the fees are added to prices for all customers.
Unless you're paying with cash at a cash-only business, you're using one of the most expensive payment methods.
The closest you can come to a low-cost payment method is a rewards credit card that provides some of those interchange fees back in the form of cash bonuses, points, or whatever, and then paying off the balance each month.
