Kartris includes support for UPS, USPS and Fedex real-time shipping systems. These use an external web API call to send weight and location details, and then the available shipping method types and their costs are returned. The open plugin architecture of Kartris means other providers can be supported if suitable plugins are crafted. You need a free user account with whichever shipping system(s) you are using, and to set up the plugin within the Payment and Shipping Gateways section of the back end.