You might as well go with a cart system as they have payment gateways already in place. Pay Pal seems to be the beginners system of choice because opening an account is free, the payment process is secure and there are several options available such as adding shipping costs and taxes at the final process. You can also accept credit cards. For a more professional approach you might want to consider a merchant account with a company such as Authorize.Net. This gives you the option of accepting credit cards without leaving your site which only the Pro version of Pay Pal offers.

There are several free carts available to you such as OsCommerce, Zen cart and version 3 of Cube Cart.

Make sure you are happy with your system before making your final decision. Once you set your shop up, it's not easy to transfer to another system. Most all carts are database driven and transferring a database from one system to another is fraught with problems, so make your choice wisely.