This barista is truly one of a kind. Inartehan pa niya eh. It's either:
a) he's being creative
b) he's bored so he tried to do something different from the usual
c) he's too lazy to make the extra stroke that would make a Y.
d) he inferred that if Joy rhymes with Koi then perhaps, it's spelled like Koi.
I wonder if he even thought about it, like "Is it Joy with a y or with an I? She looks like it's an I. So, Joi." haha. I would've been more impressed if he frenchified it by adding an E. Joie.