I don't freakin' know. After getting a rogue unauthorized bank draft for 214 smackeroos, I haven't missed sexfm for a long time. I still get the deal of the week. I do know if you get a promo 'hit' on your radio and know where to unhook the antenna when you park it, you can keep getting it for long after the promo expires. Just unhook it on the last day and wait a week or 2.
I'd still use a prepaid visa for $5 and load it for a sub. Pay them. Let it run out. And deal again. Four bucks sounds ok locked in for a few years. They need to break that one buck threshold first. With taxes!