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

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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.

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

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Begin the following day with a big bang for an optional but additional fee by touring the lively market at Otavalo, chock full of traditional Andean wares, exotic snacks and impromptu pipe performances.

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Part D is an optional insurance program that charges a monthly fee in exchange for prescription drug coverage.

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