A Little Rusty... adding a cart to an existing site?

I am returning to a site for a client after a year, who now wants an eCart added. I am a little rusty on the procedure and have set up one from scratch but never added one to an existing site. I'd like to know best practices and workflow. One problem is that I set up tables in her database that I don't recognize for data (perhaps default eCart examples?) and the other is that as I started to set up the eCart, the wizard doesn't see the same field names as in the database but has its own.

Here is what I set up last year:
an Admin area to upload images, update the database with items, etc. in DataAssist, but I did not set up an eCart or ordering system. My catalog is hand crafted with php/mysql as is my template for the design of the system. The Admin area does not as yet have overview for orders and order lookup.
My own tables in the database are as follows:
orderdetails.
orders.
shiprates.
shiptypes.
states.

Shiprates were detailed, but I don't remember adding them and since I hadn't used the cart wizard, I'm not sure where they came from. Do they look familiar? :
2 14.75 3 1 0.55 1
3 6.8 2 1 0.8 1
4 5.65 1 2 0.75 4
5 10.5 3 2 1.7 4...

I guess my question is where to begin? Are there wizards to use to set up the entire cart at this point and admin overview? Or do I just create a separate cart function and add buy now buttons to my catalog pages? What I forget and don't see how to do now is to link the database tables above to the eCart wizard and /or modify the shipping rates and types from within eCart... Or do I need to create new tables?

I hate not using something regularly, cause you forget everything and have to start back at square 1.

Reviewing the eCart feature tour, I realize checkout pages are created in the Wizard, but I'm a bit confused on how to link the processed orders to a specific database. Should the sample database table structure be followed exactly? I don't see how they are stored after checkout with paypal, simply using the eCart.

You can use your own database.

The checkout wizard has options for storing the database information. With paypal Standard, you need to create your IPN page manually to do this outside of eCart, you can find forum posts where people have done that with the details here.

Thanks Ray. This client has Website Payments Pro... I want to give her the functionality of reviewing orders, etc. For some reason this was a LOT easier, or at least more intuitive, when i used the Blue Sky basis to build the site from scratch.
Jeff