Checkout for shopping cart can be done using several ways, for example:
1. Amazon Checkout:
https://payments.amazon.com/sdui/sdui/business;jsessionid=7D5D5D0E4326C0C8DE885FB9E1D91927?sn=cba/osCommerce
2. Google Checkout with spreadsheet form (Checkout Store Gadget)
http://code.google.com/apis/checkout/developer/Google_Checkout_Store_Gadget_How_To.html
3. Paypal Checkout integration:
https://www.paypal.com/cgi-bin/webscr?cmd=p/pdn/howto_checkout-outside
4. Marchant Account for credit cards:
http://www.clickandbuy.com/US/en/merchantportal/home.html
https://www.beanstream.com/public/index.asp
http://www.charge.com/
http://www.authorize.net/
http://www.2checkout.com/community/
http://merchantwarehouse.com/credit_card_software/authorize_net
