Now, if a private company could do it for $.10 a letter, fine, but I don't see that being possible.
Here's the thing: your taxes prop up the USPS no matter if you use it or not. So even though the marginal price of sending a letter is a mere thirty-nine cents, you're still paying more through your taxes. And you really get burned if you don't send any letters, because then you get very little utility (you still get some benefit from receiving letters, but most of the cost is borne by the sender) while still paying for the service.
It's more of a free rider problem than anything else.