I don't think that this is right.
Firstly, she bought 10,656 mars bars, which were in 220 boxes. That makes 48.02272727272727 mars bars per packet. Of course, she might have bought the loose bars which were on the shelf too. But still, it doesn't work out. If there are 48 bars per box, this means 220 boxes contain 10,560 bars. So there were 94 bars on the shelf and scattered elsewhere around the store???
Secondly, she paid £2,131. This means that she bought each mars bar for 19.99812312312312 pence. This is very cheap for mars bars, no? Cue: Brit to chime in here.
Perhaps there was some promotion on, and she wanted to guarantee her spoiled little son/daughter would win. A bit like that rich father played by Roy Kinear in Willy Wonka's Chocolate Factory.
Okay. Now that I've said "Willy Wonka's Chocolate Factory" I'm going to stop.