Hi @Girish
1) Is it possible to buy TTS with several voices?
Yes you can see the different voices listed on:
Engage global audiences by using 400 neural voices across 140 languages and variants.
https://azure.microsoft.com/en-us/products/cognitive-services/text-to-speech/#features
2) Customer does not have internet access.
You can only use Azure TTS with internet connection. To use it to run locally you still have to connect to Azure Billing:
3) Can they buy Azure Cognitive service as a standalone package in this case?
I'm not aware of any possibility of buying Azure TTS in this perspective. Azure TTS is a service that you pay-as-you-go for what service amount you use.
If this is helpful please accept answer.