Drop box query

my parents are planning to come in us by the end of march
i am on depedent visa ...i am having following query

1)can they apply for drop box as my father is 65 yr
and mom 61(they are appplying at mumbai counslate)

2)copy of my passport is required?

3)in a letter submitting to us counslate is it from my hubby to them as a invitation or from me to them...