Actually, you paid karma and had to roll a red FEAT the first time you used it. The second and third times, the karma price was lower and the FEAT decreased to yellow and green. After the third time, I think you could buy the stunt for your repertoire. You could do the same in Unisystem, stating that you need to succeed at a stunt at least three times, with appropriate expenditure of DPs, before paying xp to have it permanently. Note that you are not forced to buy it after the three successful attemtps, only that you can do it.

Tchau!