They do work like gc's Ms. Harmony.
Having seen first hand how things are picked for both orders and the smokeless lounge, problem comes down to items bought before an order is pulled in the SL. Let's say they have 20 boxes of black wow cartos on hand, they sell 18 of them through the site, but someone comes into the SL and buys 10 boxes before the order is pulled. Now it's only 10 istead of the 20 that were in inventory during the times the orders were placed. So they issue store credit to get the order out as fast as possible. If you look at the in-stock level at Vapor4Life through the site you can see they're hovering around 90-95% in stock almost all the time, problem is the items they're out of first are usually the most popular. Can't go too much into detail, but remember all the inventory comes from China, that means a boat ride, customs, and if there's a mis-ship from China you're looking at a 2-4 week turn around before the correct item arrives. When it comes to testing items around the office (speaking from first hand knowledge) it's always the blanks that don't move as quickly (with a high amount of backstock) that get used.