PayPal is one of the world's most popular payment solutions, so it should come as no surprise that Drupal Commerce has a well developed and supported module to integrate with it. There are two available PayPal payment methods supported with the Commerce PayPal module: Website Payments Standard (WPS) and Website Payments Pro (WPP).
Tip
PayPal Website Payments Standard is in the process of being renamed to PayPal Payments Standard, so you may see some discrepancy in documentation and module names.
The benefit of this method is that there is no monthly cost; you simply pay per transaction for processing the payment. So, let's get PayPal Website Payments Standard integrated with your Drupal Commerce site.
Download the Drupal Commerce PayPal module from Drupal.org at http://drupal.org/ project/commerce_paypal and install it on your Drupal Commerce installation.