1. A 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 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 message package hierarchically organized 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; and program code for sending a second message to the heterogeneous application responsive to the first message.