One fundamental issue in existing reputation mechanisms, particularly those in open and decentralized multi-agent systems, is whitewashing attacks by rational providers. If identities are cheap, it is beneficial for a provider to simply defect when selling services to its clients, leaves the system and then rejoins with a new identity to avoid punishment. Current work usually assumes the existence of an effective identity management system to avoid the problem, without proposing concrete solutions to directly prevent this whitewashing behavior. This paper presents and analyzes an incentive mechanism to effectively motivate cooperation of rationally opportunistic providers in the above scenarios, by eliminating incentives of providers to change their identities. The main idea is to give each provider an identity premium, with which the provider may sell services at higher prices depending on the duration of its presence in the system. Via the use of a reputation-based provider selection protocol and a pricing model, the proposed mechanism also reduces the impact of malicious and strategic ratings largely. It is proven that if the temporary cheating gain by a provider is bounded and small, and given a computational trust model with reasonable low error bound in identifying malicious ratings, our approach effectively eliminates irrationally malicious providers and enforces cooperation between rationally opportunistic participants, even when identities are available at a given cheap cost. We propose the identity premium functions that helps cooperation sustained in relation with the identity cost, analyze the incentives of participants in accepting the proposed premiums, and discuss related implementation issues in different scenarios.