Simple OpenCart PHP/MySQL Shopping Site

My nonprofit needs a simple set of PHP pages to sell tee-shirts and a few other swag items.

I'm interested in a simple, portable solution, like OpenCart, that we can add to our current Web site, which already has MySQL and PHP installed. (We are not using WordPress...)

I don't need anything fancy, this is for a small batch of swag that will be offered during the holidays as a part of our nonprofit's fundraising. Just the basics so that we can collect orders, route payments to our PayPal, and have a volunteer monitor them and get them out the door.

Tee shirt buyers would need to indicate the size (seven), color (four), and quantity.

Other items would simply require quantity.

We'll need the users postal address, email, and (optionally) phone.

Shipping and handling will be fixed price.

Users should be given the option to donate.

Payments will be funneled through a link to our existing PayPal account.

We'll need a basic admin interface to manually track payments and fulfillment.