36509

I started off with the 3 and the 14 envelope because they have the least possible amount of pairs of numbers then I decided to do 13 and with the numbers i had left I did the 7 envelope and then the 8 envelope

in the 7 envelope was 3 and 4
in the 13 envelope was 7 and 6
in the 14 envelope was 9 and 5
in the 3 envelope was 1 and 2

AND IN THE ALL IMPORTANT 8 ENVELOPE I PUT 8 AND 0!!!

I then found out that there are other ways of doing it.
Here is one of the others:

in the 7 is 5 and 2
in the 13 is 9 and 4
in the 14 is 8 and 6
in the 3 is 0 and 3

AND IN THE 8 ENVELOPE IS 1 AND 7!!!
Iused the same principal
I did 3 and the 14 envelopes first