At check-in in SFO two days ago, I was offered to buy-up at check-in from Y to Premium Economy for $400.

I was on an AA code-share ticket. I selected a seat online 8 hours before departure with no one next to me, in front of me, or behind me. At check-in, I asked if there was still no one sitting next to me in coach. The check-in agent laughed and said the flight was now completely full in coach. Then she walked around the counter and asked if I wanted to buy up to Premium Economy for $400.

I paid the $400 and the agent went somewhere else to process my payment. Later she came on the plane and handed me a receipt.