What's Happening With Postage?

I must be dumb. I have accepted that when I buy or sell a car part and send it USPS that I will have to sell my first born to pay for it where-as big companies get a greatly reduced rate. This week I went to the local Post Office to mail five medium manilla envelopes. All five were the same size and weight but were sent all over the country, one in my own town. The price= $2.16 each no matter the distance. Today I mailed a sixth one, same envelope and same Post Office, price= $1.92. Sorry, this really bugs me. Is there something I am missing? Shouldn't I expect the Federal government to be more consistent? Help me out here, Thanks.
When I mail something to my own town, it goes 150 miles to a distribution hub and flown back to another town 20 miles away and then…