Can Commerce Kickstart be used to add Drupal Commerce to an existing site? I want to add the Drupal Commerce module to my existing Drupal website. Is there any way I can use the Commerce Kickstart dstribution for an existing Drupal site?

Seems like you're supposed to use Commerce Kickstart when you're starting to build your Drupal site, but I already built mine. 

According to the Commerce Kickstart website it says: "Anyone who wants to skip weeks of configuration to build a commerce site on the Drupal Commerce framework".

Is it really that hard to install the Drupal Commerce module without Commerce Kickstart?

I'm not creating a big store, I just want to sell 1 or 2 items.
No, you can not install an installation profile after Drupal has already been installed. What you can do is make a separate installation with Kickstart Commerce and compare configuration when you're stuck.

How hard Commerce is to install depends on you previous Drupal experience. If this is your first time around with Drupal, it will be pretty hard. But if you're already familiar with entities, views and rules, you'll be fine.