1. A tangible, non-transitory computer readable medium including program code for providing a message-based interface for exchanging information regarding customer invoices, the program code operable when executed by a processor to perform one or more operations, the operations comprising: 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 requesting settlement of business transactions related to goods and services associated with a customer invoice that includes first message package derived from the common business object model and hierarchically organized in memory as: a customer invoice request message entity; and a customer invoice request package comprising a customer invoice request entity, a business process variant package, an item package, where the customer invoice request entity includes a base business transaction document ID and a base business document type code, where the business process variant package includes at least one business process variant type entity, each business process variant type entity including a business process variant type code and a main indicator, and where the item package includes at least one item entity, each item entity including a base business transaction document item ID, a base business transaction document item type code, and a receivables property movement direction code; and 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 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 to provide consistent semantics with the first message package.