What is the cheapest service for sending notifications to Viber?

Question

API

Hello!
We have a project that requires customer identification.
Identification is carried out using SMS messages. But it happens that messages do not reach some operators - for example, to Vodaphone. Messages are short - 4 digits.
We wanted to implement sending notifications to Viber.
But we did not expect to see such a price. All the options that I saw are at least 200 euros per month, while unused funds are burned out at the end of the month. We do not yet have such volumes of messages.

What are the options? Are there inexpensive mailing services without such conditions?