I am not sure if the PS+ 3 months extra promotion is actually still going on. If you are concerned about not getting it with a PS+ prepaid card, then I would suggest getting a $50 PSN card so the person can just put that cash value on their account and purchase the year of PS+ straight through the PS3 with that amount.
Unless, that is, you are afraid they may use the money for something other than PS+ =/
Just found another thing I dislike about the new forums.. The "Correct Answer" can only be on one response where I think all three here are correct..
Well, I wanted to "force" (LOL) this person to get PS+, so I went ahead and did it. We will see in the next few days how it works out. Hopefully it's works out like the Gamestop one..