Adequate IT support for Business-to-Business integration(B2Bi) is indispensable in today´s globalized world. Agreement among personnel from different enterprises as well as distributed computing issues are major challenges to the automation of B2Bi processes. These challenges can be addressed by applying the choreography language ebXML BPSS (ebBP) for declaratively specifying B2Bi processes and using Web services and WS-BPEL as dedicated integration technologies. ebBP Business Transactions (BT) are the primary building block of ebBP choreographies and specify the exchange of up to two business documents in a declarative and technology-agnostic way. Composing BTs within choreographies and realization of QoS raise important requirements for the orchestration layer. This paper investigates these requirements and presents a composable, abstract, flexible and QoS-aware execution model that can be implemented using Web Services and BPEL.