Its jut regular Postal 1 there is no add on for the extra levels. :(

I haven't installed it to check this, but for something like this I would advise you to contact GOG support. They assisted in getting the recent update to Postal 1 uploaded - the update contains a number of fixes for issues that have been in the game for quite some time. If it is missing it might be an oversight.

EDIT: Just to clarify, when you played through, did you have the four levels from the mall to the holiday hotel resort before getting the schoolyard end level? If you did then you did in fact play through the four additional levels from the Special Delivery expansion.

Will try that, I remember the last one let you choose which you wanted to play.

Looking into it, the add-on should be there but seems went missing when we pushed the last updates.

As I explain in my post here, the original poster of this topic, Cheater87, is mistaken. Postal still contains all of the "Special Delivery" expansion's additional maps - they're simply tacked onto the end of the original 16 maps. However, Cheater87 is correct in pointing out that the player no longer has the option to choose which version to play.