Despite the deceptively long name, WordPress Simple PayPal Shopping Cart (WPSPSC) is exactly what it claims to be: a simple shopping cart integration for WordPress that allows customers to purchase items using PayPal. It actually takes longer to describe how to use it than it takes to learn and master it. This plugin is free.
For the record, WPSPSC does have a bigger brother called WP e-Store (http://bit.ly/e-store) that allows for the sale of both physical goods and digital downloads. The current price is a very reasonable $29.95.
Don't expect a long feature list here—the key word is simplicity. WPSPSC follows the philosophy ...