Looking at the fact that you are at level 40, you must have a high reputation level. You can go around and collect the lost mails in cities of neighbors to get stationary knifes and then use them to explore the lost mail around your own city. They will drop some items sometimes. One that drops most frequently is the 'letter of hopes', which can also be got b collecting from post offices at a better rate. The others are of a collection, completing which gives you a golden postage stamp, valid for 5 minutes.
You also need to fly to Fiji, Madagascar and Easter Isles with the letters of hopes to complete the second quest, each of which you will have to collect 4 times.
GOLDEN POSTAGE STAMP allows to land postal guest planes for 5 mins. You have to land 20 of them. So, you will have to gather as many GPS I mean golden pos...... not the navigation gps. Land a plane collect profits, visit a neighbor, come back to see the plane gone and another waiting. Do not refuel or give passengers to the plane.