Common summertime problemwith velour seats.

Top Tip: Hold onto something metal (I hold the top of the door) before
you put your foot out onto the ground. That way the discharge will go
through the sole of your shoe, and you won't feel a thing.

Any object can carry static provided it is isolated from another object.  In
this case it is the seperation of you from the car seat that builds up the
charge between the 2 bodies.  The car will probably dissapate most of its
charge to ground via the tyres, but your charge generally varies on the
conductivity of you shoes.  Trainers with polymer soles generally insulate
very well as do industrial type soles (like DMs).  Thinner soles,  leather
or older dirtier shoes generally conduct better.  So by the time you go to
close the door you are charged and the car is a a different voltage probably
differing by 1000s of volts (10s of 000s usually).  The amount of charge
generated will vary according to the humidity, as will the conductivity of
some materials, so drier is worse generally speaking.

The usual trick as suggested by other posters is to hold onto the door as
you get out/off the seat, this insures you are at the same potential as the
car and charge is dissapated through the car to ground as it is generated.
The paint is not insulative enough to resist breakdown above a few 1000
volts.

One way is to work out which shoes it is that cause the problem and avoid
them when driving.  Another way is buy conductive footware used by various
industries usually from catalogues.  You could also wear a heel grounder,
but these are uncomfortable.

It is rather sad that shoe manufacturer pay so little attention to this
uncomfortable problem as a very small additive would be enough to spoil the
highly insulative materials characteristics.

--
Tony Stanley                   ++Always Learning++