pep talk
You will be fine. I suspect the only problem you have will be keeping the readings short enough.
Have a timer. If your readings are going to be fifteen minutes, set it for thirteen. When it goes off keep reading a couple more minutes in a summary. Turn over the cards and put the deck back together when the timer goes off. Do your summary with no face up cards. That way the reading comes to a clear end.
Don't ask them if they have any more questions. In a longer reading you would but in short readings this is asking for trouble. When you finish the reading thank them and you are done.
If you have some nice, inexpensive rocks bring them. It is a good way to finish a short reading to give them something as you thank them..."Here, I think this rock feels right for you". You can also give them a business card or other give away. A gift brings the reading to an END.
The two card idea is very good. YOU shuffle...just let them cut once. Take the top cards of each pile and turn them over on their respective piles. If you let them shuffle you will get way slowed down...some people shuffle forever.
If you can set things up on a tall table, like a small round bar table. You have a chair, they don't. For a short reading they can stand. Some people are really hard to get out of a chair once they are in it. This may not be possible but take a look and see if you can do it.
Set up your booth so they are facing the line and can see the people waiting. That will keep them moving along a bit better.
You decide if you want the death card in there...in a short reading this can make things too interesting. There are people that are passionate about both sides of this idea.
Remember, your doubts are only in your head. People coming to you will know you are a wonderful reader because you are there...that's all you need to remember.