Well, I got a chance to try the Cha-yen (cardamom) last night. I've never had authentic Thai tea, but this wasn't what I was expecting. It was a flowery tea to me without the sweetened condensed milk. There might be a chance that if you add a sweetener option, it will be like the Thai tea that most are used to, but I'm not sure.
I always recommend shooting an email to Wlad before getting adventurous with their flavor list. He will guide you in the right direction and may have suggested sweetener had you done so. My experience with their juices is that they steer away from sweetener by default to allow the authenticity of their hand made extracts come through. This is especially true with their teas. The only tea I've had from them which came out of the gate very sweet was the Chai Latte (amazing). If you haven't already, shoot them an email with your likes and dislikes so they can build a profile for you. After some time they will apply your profile automatically to your orders and tailor your juice to your taste. Keep that juice and buy some liquid sweetener. Add a drop at a time to the bottle until it meets your liking. I've done this to several juices and it made a big difference without having to give up on a juice.