1. A non-transitory computer readable medium including program code for providing a message-based interface for exchanging information for point of sale transactions, the medium comprising: program code for receiving via a message-based interface derived from a common business object model, where the common business object model includes business objects having relationships that enable derivation of message-based interfaces and message packages, the message-based interface exposing at least one service as defined in a service registry and from a heterogeneous application executing in an environment of computer systems providing message-based services, a first message for providing a notification about a bundle of created or updated point of sale transactions that includes a first message package derived from the common business object model and the first message hierarchically organized in memory as: a point of sale transaction bundle notification message entity; and a point of sale transaction bundle notification package comprising at least one point of sale transaction bundle notification entity, where each point of sale transaction bundle notification entity includes an external identifier (ID), a type code, a business transaction date, and a company ID; program code for processing the first message according to the hierarchical organization of the first message package, where processing the first message includes unpacking the first message package based on the common business object model; and program code for sending a second message to the heterogeneous application responsive to the first message, where the second message includes a second message package derived from the common business object model associated with the first message package.