No, it is not mandatory to put each letter in caps, letters can be used in mixed case as well.
A Promo code can be entered is all caps as well as mixed case in order for it to work. it will be valid as long as the correct letters and numbers are present regardless of case.
It is not possible to make a Promo code case sensitive in order for it to work, it will be valid as long as the correct letters and numbers are present regardless of case.