Seems odd to have a public postal service and still require people to buy stamps for votes.
In this country, ease of voting is not prioritized.
Edit: Apparently there is a commission set up. It's part of the executive branch, but still, the USPS can't up and decide the price of a stamp: https://en.wikipedia.org/wiki/Postal_Regulatory_Commission
I don't, only saying that the bit you're commenting on is wholly irrelevant.