SagePay is a popular payment gateway based in the UK, which can process credit and debit cards much like popular providers such as PayPal and Authorize.net.
Download the Commerce SagePay from Drupal.org at http://drupal.org/project/commerce_sagepay_form and install it on your Drupal Commerce store. You will also need a SagePay account set up, which you can do at http://www.sagepay.com.
Once you've installed the module, follow these steps to enable SagePay for your Drupal Commerce store:
Navigate to Store settings | Advanced store settings | Payment methods and locate the SagePay module in the list:
Click on the edit link to alter the module's settings, and then click on the edit link once again under the Actions block:
Under the Payment settings block on the next screen, enter your SagePay Vendor Name, Encryption Key, and Vendor Email associated with your SagePay account:
Next, the Order Description field...