United States Postal Service [USPS]

I went to Catalina Station Pasadena CA 91106 Post Office to mail a package to return an item to a retailer in UK. Clerk #31 served me, she weight my package and put labels on it. The retailer required me to ask a stamp from the post office in the counterfoil as a proof of posting, I dont know what it is for but I asked the clerk politely to stamp it for me.
She said she could stamp it for me for another $1.50. I asked is there any way for me to track the package? She said no unless I pay another $11 for tracking number. And then she offered me again to stamp it for $1.50, and I asked what stamp was it? And she said it was a meter. I asked what a meter is for. And she got impatient and she said she only had 2 minutes left before she closed the store (no one in line at the time, only me, her, and 3 other clerks). She got mad and started to take off and peel all the labels she put on my package and yelled "IM GONNA TAKE THESE OFF, YOU GET OUT OF HERE!" I asked her not to ruin my envelope, and she asked another clerk to peel those labels from my envelope. And I ask her if I can talk to the manager and she said in high tone "I AM THE MANAGER HERE, YOU GET OUT OF MY STORE!" I asked the other clerks what her name was because she didn't have her name tag on, no one gave me her name (I know her clerk number is 31 from a receipt of another package I sent right before this happened). She told me to get out of "her store" again and again. She said that a few times in an extremely rude tone until I left.
I've never been humiliated like this. this is the most horrible customer service experience I've ever had.