@AmandaC Hello there, just want to say that I've moved your post to our Prepay board. I'm guessing that you have a Prepay plan based on your post. Let me know if that's wrong .
I'm here to help you but I will need more info. Were you using the app or website version of My3? What error did you get? Have you ever successfully applied a 500mb add-on?
@AmandaC that's weird! Is there an existing 500MB on your account right now? By the way, what's the name of your plan?