When do we pay the bus driver?

So it isn't a "one size fits all" sort of answer. If you are on one of the bigger buses with 2 doors, it makes sense to pay when you get on, and get off from the back door; but this can change with the driver's preference. On the combis it really doesn't seem to matter whether you pay when getting on or getting off. On the bus to Petatlan or Barra, it seems more usual to pay when getting off because the fare will depend on how far you rode. As mentioned, it sometimes changes if the driver has a helper who may collect during the ride. I just always pay when I get on, as do most of my friends and neighbours who take the bus more often than I do.