The store made a guarantee for promotional purposes, obviously.
Unfortunately, the people in their marketing/promotions division failed to realize that the 17-year-old kid behind the customer service desk is not only very unorganized, but also couldn't care less about getting things done in a timely fashion (I dont meant to rope all teenagers into this category, but this particular branch is clearly working slower than expected). Because of this, the OP is getting something at a discount.
The OP is holding the store to their policy, which everyone should do, and perhaps the manager will question why the store has been giving these discounts out to everyone. He or she will retrain his/her employees and motivate them properly, in order to meet the 1 minute criteria.
The OP has done nothing immoral or illegal. Its not like he is demanding something for free, out of the blue, because the service was so poor. He's simply taking advantage of what has been given to him through poor marketing on the part of BB.