I agree with many others here that the mystery box feels like a rip off because at the end of the day you may end up with no cards that you want.
What, if anything, would those of you who do not like any randomly selected cards in the Friday special offers prefer instead? Without this aspect the special offers are pretty much equivalent to a permanent price reduction - they would reduce to "buy your favourite cards cheaply", which we would all like but which is hardly practical from a business point of view. Totem, much to their credit, do occassionaly make some offers of that kind, but it can never be the norm unless they are overcharging outside of the offers.
I don't think people expect a discount on all cards every week that is a special occasion kind of thing.
Maybe something more like has been done previously where there is a discount on a specific catagory eg. bikinis. I know on a few occasions I have bought cards that I would normally have bought because of this type of offer and I've really enjoyed them.
There is also the type of offer like last week where if you buy X number of the weeks cards you get one free next week. This could work even if it was an older card or maybe one from you wishlist.
Some people (like me) value the choice over which cards they have in their collection and getting random cards doesn't offer that choice.