I am booking from the US.

When I pay for the increased fare price allowing me to select/request seats it shows all seats as either available or not. I can go all the way to the payment process booking exit row or front row seats and the price does not change. I held my booking and returned the next day to the website to pay for my flight.

Upon returning to the site I found my price was still the same. However, the seats around me that were open were now marked as "premium" or something and had an additional $100 surcharge. When I changed my seat, then tried to change back, the seats I had selected were now showing as subject to $100 surcharge.

How does this work? I selected these "premium" seats in an exit row and a front row initially and it said nothing about an extra cost. I could have gone ahead and charged the flight, but what would have happened? Would they have assessed me an additional $100 at check in for my seat selection, or are all seat selection surcharges collected at the time of initial reservation, assuming you retain the same seat you request in your initial reservation?