Should You Tip When Picking Up Take Out?

This is one of those situations that I wonder about, every time that I pick up a to-go order. Obviously, if I'm dining in, and there is a server, I tip. But what if I've ordered & paid online, and I'm just running in to pick it up...To tip or not to tip? This has stirred up quite a conversation in the Tri-State Restaurant Reviews group on FB.

If you do tip, what is the fair amount? Honestly, I'm clueless about this, and I'm not going to ask someone what they make per hour, and base my tip off of that. I feel like under tipping is almost as bad as not tipping. If you work in food service, please share your thoughts!