It seems like a novel idea, and I think it should be given a chance at least. It is a nicer way of implementing a UED, or even Jason's Magical Christmas land idea. Spending a pick to get a pick from a larger pool later seems interesting rather than just adding more cards later.
My take on the 'shop' would allow for three colour cards (charms seem a good place to start) and any of the really narrow build arounds (
burning vengeance,
aluren,
enduring renewal, etc.). Have it known at the start, so then if someone gets the first shop pick, they can see what narrow build around seems to be open and go for it. Having that many options, even if your draft doesn't end up supporting the card, you can throw in a charm instead or something.
The most important part about UED and the 'shop' is this:
Can't have anything that you would want to take over a normal card in your cube.
This should be true until there are (close to) no cards in your colours in that pack. If you break this rule then the shop picks basically become freerolls. They are almost exactly like the charms that you want to put in the shop. They get their power from the flexibility. None of the options on the charm are great for the cost, but being able to chose the right thing at the right time makes up for it. I would keep the power of the cards VERY low in the shop, just to be on the safe side for the first runthroughs. If it doesn't seem to get any love then slowly increase the power level.
My way of implementing it would be using some of the worse conspiracy cards and just sharpieing over them. It would be nice to know when someone as taken a 'shop' pick. Might make it more fair, by making it known info since it can be more than one card anyway.
My pick for the best card to sharpie over would be this guy.
I mean you are going to barter with the shop aren't you?