They should just give 20$ to each 100$ there is nothing stoping anyone to make multiple accs and spend 100 in each...
They could simply just give 20$ to every 100$ already, as you can do it on your on, just for the sake of not changing accs... though I don't think I will spend a dime on it.
I know its only $20 but if the EU had a similar deal I'd probably take it up. There's loads of content I want to buy on the PS Store but I figure it can wait. Get a wee bit of cashback and it'd be worth it.