Note the weasel words "may" and "if eligible" in
You may get 20% off with PayPal if eligible.
This also means you may get NOTHING, 1%, 3% or possibly a sharp stick in the eye.
Did you actually receive the 20% from your film purchase or just simply the promise to have the 20% sent to you at some unspecified future date? As I read the terms and agreements it seems, as it does with most rebate programs, that they make no solid promise to pay. Just to do their "best" to get you your money. And with Microsoft, doing their best isn't all its cracked up to be.
That said, I hope this does work and I'm going to start looking for the little magic icon in auctions!