Well, I bought a cake there. This is a famous shop and many tourists like to buy food items as souvenirs here.
There were three staff at the shop. One was manning the cashier while the other two were busy packing food items into small boxes with their backs facing the counter.
While queueing up to pay for my cake, a lady walked in with a cake in her hand. A staff with her back facing the counter saw her through the big mirror on the wall. Knowing the staff saw her, the lady asked if she could change the carrier that was used hold the cake.
The staff said "The paper one?" in a half questioning and half confirming tone.
In a second, the staff plonked a paper bag on top of the counter and turned back to continue with her packing.
As the paper bag is folded, the lady had to open the bag and transferred the cake by herself over the counter before rushing off to the departure area.
I guessed she could be buying it as a gift and thus came back to change for a nicer carrier instead of the flimsy plastic bag.
What if the staff had asked the lady if she's buying it as a gift and put it in the nicer carrier in the first place?
And I believe the mirror was put there so that the staff can see customers coming in when they're busy packing with their backs facing the counter. Then they are supposed to turn around to serve the customers. Not to talk to customers through the mirror.