All rewards are unlocked by sending notes.

Reward	How to Unlock
Buy stationary	Send 15 Notes.
Cycle between notes with L and R buttons	Send 25 Notes.
Download photos and voice clips	Send 10 notes.
Edit the date stamp on notes	Send 20 Notes.
Send photos	Send 3 notes.
Use 3D Pop-up text	Send 4 notes.
Use stationary	Send 5 notes.
Use voice recordings	Send 6 notes.