A country should IMHO guarantee its residents access to mail. And nowadays, internet and phone lines.
But wouldn't it be more equitable (people will send different amounts of mail) and more utilitarian to just directly transfer the cost of the subsidy to everybody below an income or wealth threshold?
Instead of the government saying "you can only spend this on mail", let people decide whether they want to get access to mail, or buy other things.
The problem here is that you're taking for granted that people would want to spend that money on mail rather than other things.
No.