In each level there is a red parcel to collect, 18 in all. After you find the parcel, you must also find the mailbox to mail it back to the college. When you are in holding the parcel in front of the mailbox, press to mail it off. Locate and mail back all parcels for this achievement. See this thread for location details.
Or, if you're interested in doing it a slightly cheaper way, in the Classroom in the college and go to the chalkboard and enter in the cheat codes. They automatically unlock the parcel and the extra, and will get you both this achievement, and "You Cheat very big!" and none of the cheats at all in any fashion impact the obtaining of achievements. Follow this link for the cheap, cheating codes.