Incentives to promote customer-to-customer helping behavior have already been attempted in various forms by a considerable number of firms. Recently, some firms have gone a step further by offering incentives not only to the helper who provides assistance but also to the helpee who requests it, and some have even introduced incentives that account for other users' utilization levels (a form of externality). Therefore, there is a need to explore desirable directions for customer helping behavior promotion incentives by comprehensively considering externalities as well as both helpers and helpees. The problem is that existing research on customer helping behavior promotion has not incorporated the externalities enjoyed by general customers other than helpees into its models, thus limiting the derivation of implications for diverse real-world situations. Departing from this awareness of the problem, this study illuminates desirable forms of customer helping behavior promotion incentives after incorporating the externalities enjoyed by general customers other than helpees into the model. The main findings of this study are as follows: (1) Service firms can achieve greater profits when providing incentives to both sides rather than only to the helper. (2) As the externality of utility enhancement for general customers from reactive helping behavior between helpees and helpers increases, it is desirable to raise the incentive levels for both helpers and helpees. (3) As the externality of utility enhancement for general customers from reactive helping behavior between helpees and helpers increases, it is desirable to increase the proportion of incentives allocated to helpers. (4) As the externality of utility enhancement for general customers from reactive helping behavior between helpees and helpers increases, it is desirable to increase the proportion of performance-based rewards within the helper incentives.