It's an extra step for them to have to take, each additional step acts as a slight deterrent- unfortunately the same is true for paying customers. chances are sales will be missed because people can't be bothered to go find someone to open the door for them
I also wouldn't be surprised if it contained one of the less popular Dimensions fun packs (those that have been regularly available for £5)
typically the mystery boxes like to me to state that you are getting the items at around half RRP - so a £15 RRP in the Dimensions set helps to boost the % discount of some other items.
I was tempted to buy one, but I've had a couple of mystery boxes in the past and generally when I've gone through he items properly they just about break even at giving me the price I paid worth of stuff I'd actually keep - meaning that the items I keep are around RRP to the price I paid.
Considering this didn't have anything claiming an exclusive item the decision not to buy was easy, chances are anything I want from the box could be gotten elsewhere for less than the total. I might be proven wrong when people get their box and post the content, but until then I'm fine having not bothered.