whats the difference in balloons? Military question kinda? not sure where to put...

[Butterfly]

...it...lol? i wanna send a care package to my military fiancee for his birthday and i really wanted to send a box with a balloon in it. (I'm sending a different box with his gifts.) i think it took about 3 weeks last time for him to get his box because he is in japan right now. my question is i'm not sure what kind of balloon i could buy or if it matters. i spoke to a friend and she suggested i buy a mylar balloon. I looked online but im kinda confused about what to get please help! thanks