SEA-SFO few days ago went out with 2 empty seats. C+ was almost empty. My seatmate wanted a window seat and downgraded himself to C+ after dinner, so total 3 empties in F on landing
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Coming back, there was only 1 F seat available 24 hours out, but I wanted earlier flight anyway. Earlier flight was completely sold out in Y, but had several F seats selling for $900+ each. I paid $63 upsell to get remaining F seat on my flight and SDC for free into $900 seat on earlier flight.