So why do they charge for their service if it’s not for profit?
Not-for-profit != do everything for free
Not-for-profits still charge for their services.
Why do they keep raising their prices? Well... let's go back to 2006 when the 109th, Republican-led Congress (Senate and House) passed a law requiring the USPS to prefund 75 years worth of retiree health benefits / pension ($110 billion) over the span of 10 years. That's why.