[digi_owl]

There may also be the physicality of it.

A t-shirt is something you get, and can keep as long as you don't ruin it.

BTW, i recall reading that someone had more success with a "unlock feature X" model of payment, even though it in total was more expensive than unlocking everything at once.

[Cerium]

For a great example of that look to the Android app Pleco, a Chinese dictionary. Free to download and use, but have to pay to unlock features. Between 5 and 30 dollars per feature, I've sent at least 70.