please help me see the big picture

zencart 1.3.9

I successfully installed zen cart on my server and i have configured my business paypal acct and entered the api info from paypal into zencart.

what i don't understand is how the express checkout integrates with zen cart and my site.

do i use the paypal express checkout button and somehow vector to zen cart

does my user see zen cart instead of paypal checkout - if so how to i set up an interface on my user's site to get to zen cart

i know i have 3 zencart php files that have to go somewhere but i don't understand where?

thank you very much!

one more note on my application:

i already have a site built and i ONLY want to use zen cart to control tax and shipping which paypal doesn't do well. This site only sells one product.

Zencart isn't designed to be a back-end cart script for an existing site. It's meant to be a complete shopping cart program. Your customer will need to be sent to Zencart in order for you to use Zencart's tax and shipping functions anyway, so it might be easier to just create a new site in Zencart to replace the existing site.