I'd agree with most other comments so far. What's wrong with a plain brown or white envelope ? Advertising what's inside doesn't look like a good idea to me ! Maybe the name and Post Code of Giffgaff displayed, but a bit more subtle - like on the back ?
Another vote for a plain simple envelope from me.
But if someone has ordered a sim wouldn't they be expecting it, so why would it be confused with junk mail?
Unless you're the only person at your address, there's at least 50% probability that a different person will see the mail before you -- and it's that person that needs not to think that it's junk mail. Even if everybody that orders a sim tells everybody else that they're expecting a sim, I doubt they mention they're expecting it in an envelope that looks like junk mail.
Most junk mail doesn't have an address on it (unless it comes from talk talk in which case it has an utterly stupid address that isn't yours anyway) so they don't bother to look to see if its addressed to anybody.
And note that many senders of junk mail go out of their way to make their junk mail look plain and unidentifiable, which suggests they think there's a greater probability of the envelope being opened by the right person that way.