When at a M.A.C counter, how to buy a lipstick?

I was at mac the other day for the second time, the last time i knew what i wanted and just asked the lady to get my colour, bought it and left. This time i wanted to choose between a few lipsticks, once i chose the one i wanted to buy from the testers on display, i didnt know where to get the actual product that you took to the till to purchase ... this is quite embarring please tell me what to do