How to save on postal costs.....

This sheet of stamp I bought to mail a packet to my daughter. She and her husband are expecting a baby next month and my wife did make some things we mailed to them. How does it works ? For one pack you can use a special non stamp thing to pay the cost. You have to pay € 6,25 for postal rate. I did use this Scoutingsheet (5x 2 different stamps) total € 7,20.

Now I alway's get my stamps back from family(so alway's use good stamps....) so these also came back. I put this item on EBAY just to see what happens. After 7 day's it was sold for € 9,00 ........!!??? Very nice for me because free shipment and also a small profit on the stamps...