(From my article for this month's A&E)
In games that don't try to simulate every possible detail, shopping falls into three varieties: Routine, Difficult, and Extraordinary.
Routine shopping is that which is so basic, uninteresting, or unimportant that it's not even worth the effort to play out. If I want my main Cthulhupunk character, James Firemaker, to get a new gun, or Lisa decides her Hub character, Dr Martius Lecks, would have picked up new medicines, we don't bother to play it out—it is just specified to have happened. Even items your PC doesn't always carry can be Routine—if all you have to do to acquire it is say you're doing it between sessions, or specify it was done in-session, it's Routine, even if you have to make a roll to see if you had the presence of mind to do so. If Firemaker shows up to visit his grand-daughter, and hands her a picture-book that was never mentioned before, that was Routinely shopped for—where it came from, and any effort involved in its purchase is unimportant. The most important aspect of Routine shopping is that it is never played out.
Difficult shopping is not important or interesting enough to be the focus of a game session, but is played out, at least to some degree. When Jealousy, my Hub PC, took delivery of her new ship, the Monarch, that was an example of Difficult shopping—there was no adventure that was the "Jealousy gets a ship upgrade" adventure, but the ship was delivered in game, as a result of in game events (in this case, her getting an influx of capital from the previous couple of adventures). Likewise, in the same game, when Stephen Tihor's PC, El Thorndike, decided that the other characters were both underdressed and under-armored, this was not Routine shopping—it was worthy of being played out and being the background of an adventure—but it wasn't the goal or point of an adventure itself either, and if the characters spent an adventure doing nothing but Difficult things, shopping or otherwise, the game wouldn't be much fun. If Firemaker goes shopping for the perfect picture book to give to his granddaughter, Mali, considering several possibilities and agonizing over his choice, before happening on the actual plot (which might involve the cashier at the bookstore, or interrupt the shopping expedition), that's a Difficult shopping expedition. Difficult shopping is that which is worthy of plaything through, or at least playing at all, but which isn't the main plot of a session, instead, it's flavor, background, and/or complication.
Extraordinary shopping, by contrast, is plot. When the PCs go on a quest for 4 ingredients, or 3 magical artifacts, or spend a session bargaining with Hades for the artifact they need to Defeat the Dark Lord, that's Extraordinary shopping. It's hard to imagine a Firemaker Mali Picture Book adventure, but I suppose one could be imagined, if, frex, he was looking for a very special picture book, one that would teach her the things she was destined to need in her later years, and had to go through quite a bit to finally get it. Extraordinary shopping (and Extraordinary events in general) are what makes a roleplaying game adventurous, and in general, every roleplaying session should have at least one situation that is Extraordinary for each players. This can be the same, shared Extraordinary situation, or it can be some combination of separate ones that apply only to a subset of the players/characters each, but cover the group as a whole when taken together, but in my opinion, a game that doesn't have something each player is involved in and considers Extraordinary is a failure (one could extend this out to campaigns, I suspect, and claim that an extended campaign should have it's own Extraordinary sessions which act as the crux and motivation for the more ordinary ones, but that's outside the scope of this article)—because such a game is by definition missing plot, that which is going on and driving things, rather than merely happening. And that's the point, really—what distinguishes Extraordinary shopping is that it is the plot.
So there you have it. Routine shopping might be worth a roll to determine whether it happened or was successful (or not) but isn't played out. Difficult shopping is worth playing, but isn't the plot, instead, it's subplot or background. And Extraordinary shopping is plot. As should be obvious, at this point, I could replace "Shopping" with nearly anything, and it would still be valid; the theory still applies if it's changed to combat, medicine, eating, or nearly anything else—the events of roleplaying games divide into those that are unworthy of even being described or played out, instead being determined by fiat, those worthy of being played out, but ancillary to the main event—that which exists not as plot, but in contrast to it, and those that are driven by and the high points of plot itself. All three, really, are necessary: Without Routine events, the game is solipsistic, with nothing happening at all unless it's on screen. Without Difficult events, there's nothing to contrast the plot to—it's all meat, no flavor, and little opportunity to just play. And without Extraordinary events, without plot, there are no highs at all, no adventure, nothing happening except the PCs going through trivial, textureless actions which may be entertaining, but aren't exciting. Without any of these, I, at least, find myself in a shopping expedition of a fourth type—that of shopping for a better game.