We went to Bali in October last year and DS was about 19 months. By that age he no longer had formula, just cow's milk at bed time.
Be sure to use bottled water in the kettle to wash the bottles with.
We gave DS UHT milk from the Bali supermarkets. If you are still using formula, then yes I would prob go with the tetrapaks for ease of use.
Really, in the heat over there, DS was more than happy to have his milk bottles replaced with water more often than not!
HTH