I need sage advice on adding a shopping cart to my site.

I am selling physical products.
Much of my site was created using php/codeigniter.

I have spent most of my budget already.
I was thinking it would save me some of my hard earned cash if I could find a script that could work and tweak anything that needed to be adjusted.

I was thinking that I might use PayPal to start.

can you point me in the right direction?

Matt