Solution

Sealed Solution Maryam

First name
Maryam
School
Hollywood
Country
Age
6

I first sorted the sums from biggest to smallest. I tried different combinations of digits that add up to the envelopes' numbers.
I also made sure to use bigger digits sooner than later.

I created the following table:

14 = 6 + 8
13 = 9 +4
8 = 5 +3
7 = 7 +0
3 = 1+ 2

While I was solving the sums, I realised some combinations of digits was not possible as I had used them earlier.

For envelope 8, the number choices are 5 and 3.