Multi-item, multi-unit negotiations in industrial procurement pose serious challenges to buying agents when trying to determine the best set of providing agents' offers. Typically, a buying agent's decision involves a large variety of constraints that may involve attributes of separate items as well as attributes of different, multiple items. In this paper we present Ibundler, an agent-based service offered to buying agents to help them determine the optimal bundle of received offers based on their constraints and preferences. In this way, buying agents are relieved with the burden of solving too hard a problem (the problem can be proved to be NP) and concentrate on strategic issues. IBundler is intended as a negotiation service for buying agents and as a winner determination service for reverse combinatorial auctions with side constraints. To the best of our knowledge this is the first agent service for such type of complex negotiation scenarios.