Aren't Twitch subscriptions supposed to cost $4.99 per month?
When subscription buttons were first added, Twitch experimented by offering non-standard sub-options such as $2.99 subs and one-time subscriptions.
While Twitch no longer offers one-time sub fees, partner channels that signed up were allowed to continue offering one-time subscriptions.
It's worth noting that Twitch Prime subscriptions can be used for channels with one-time sub fees.
List of channels with one-time subscriptions: