They are, and there's another way that they increase prices: by changing rules.
I remember a couple of years ago the post office representatives came to my office to tell me about this great new rule. When I looked at it, they had changed the sizes we were allowed to mail to be slightly smaller than a newspaper. If you were slightly bigger than that new size, you had to pay more. Our increase that year was 9.8%.
We get at least a 5% increase every January from the post office.