Cái gì, from what I have read, is to put emphasis on the “what” in the statement. However, if it’s just general speaking, the “cái” is not needed to get the point across.
What was wrong with: 'Cái gì bạn muốn gửi?
In Vietnamese, questions are usally made by replacing the corresponding word in statement.
Eg: Tôi muốn gửi một bức thư. --> Bạn muốn gửi cái gì?
Why cant this question end in không?