This research examines the effectiveness of an issue-packaging agenda in complex multi-issue negotiations, drawing on mental accounting theory to explain how negotiators structure and process decisions. We propose that issue packaging, by structuring negotiations into manageable subsets, enables negotiators to handle a high number of issues while preserving flexibility for integrative trade-offs. Across two experiments, we compared issue packaging with a simultaneous agenda, a strategy that has proven effective in negotiations involving a moderate number of issues. Experiment 1 showed that issue packaging outperformed a simultaneous agenda in negotiations involving many issues, particularly when applied consistently across negotiation phases. Experiment 2 demonstrated that these benefits emerged only when issue packages remained open until a final agreement was reached; premature closure curtailed flexibility and impaired across-issue coordination. Mediation analyses revealed that the advantages of issue packaging were driven by enhanced judgment accuracy and logrolling. Together, these findings show that structured yet adaptive agendas improve negotiation performance by balancing cognitive manageability with flexibility for value creation.