No, this was all on the up-and-up. It was at check-in, not the gate. You get an Avianca receipt and re-issued boarding pass with "paid upgrade" written on it.

The pricing has been consistent at JFK ($230 in Jan, $179 in Feb) but it's been all over the place in BOG and MDE - on Monday I paid $120 for BOG-JFK and the week befor I paid $230. And when I paid $230 the front was empty and when I paid $120 FC was full. Go figure.