The following in themselves do not affect the class of travel:

the purchase of goods and services on board (whether by way of a supplement), unless those goods or services amount to an upgrade
paying extra for the right to choose a seat (so long as it is not a better class of seat)
paying extra for a seat with extra legroom, provided that seat is not separated from the ordinary seats in any way and provided there are no other benefits associated with the seat
paying extra for benefits such as pre-booking, reduced check in times, fast track through security, priority boarding, access to VIP lounges, transport to or from the airport, superior baggage allowances
paying extra to be seated next to an empty seat
paying for an empty seat