Best way to have a cart..

Hi again,
Seems like nothing can be simple anymore, so many options.

I have a client that has a static website with planners, very little options besides the type and binding.
They now want to put a cart on there so people can buy instead of just advertise the products.

What route would you go from static to cart?
Use links for paypal/google/etc checkouts?
Use magento/oscommerce/etc and try to integrate?
Go to CMS and rebuild site with commerece (zencart/joomla addon/etc).

They would never be adding their own product, and maybe 1 time a year I add/remove one or two products.

Thanks for input!

If their products stay static for the most part and they are happy with only accepting via Paypal or Google Checkouts, I would probably just use a static system still and add a few links.

If they are happy with you rebuilding the entire site and want to pay all of that extra $$ for you to do it, then that may be the way to go -- but to me it does not sound necessary.