Hi, the other day I had a look at my iStock account and clicked on the referrals tab. There is now a higher commission for referrals of $20, but there are now two other options of 20% of the value of the sale or 20 credits.
iStock, being the lovely people that they are, have set this to 20 credits by default. Just a heads up that if these are not any use to you (as they are not to me) then you should change it to something else soon.
Cheers