Optional fee-based services offer more extensive monitoring of credit information, personal identity records, social security Numbers and online transactions.

你还可以选择收费项目，获取对信用信息、个人身份记录、社会保障号码和其他网上交易活动的更全面的监测服务。

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The company hopes one day to make money by offering users optional fee-based services from others.

WSJ: A Free Service to Store Medical Data in Case of an Emergency

Nina also includes a range of existing Nuance text-to-speech voices, and Nuance can develop a custom TTS voice for an optional development fee.

ENGADGET: Voice assistant 'Nina' lets any app obey commands, makes speech your password

Instead, Southwest added an optional early bird fee which allows travelers to check in earlier to get a better place in line.

FORBES: SAPVoice: Swarm Intelligence: How The Behavior Of Ants Can Be Used To Improve Everything We Do.