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When you double-click an item in the left or top-right pane of the shop, it's instantly purchased. I've always purchased by double-clicking, it's quick and there's no disconnect between selecting and buying that would allow my brain to fail and buy the wrong item. In the bottom right section though, where an item is displayed once selected, you can't double-click to buy (although that hasn't stopped me wailing away with my mouse button on it). In the interests of consistent UI design, it would be great if it was changed so that items could be purchased by double-clicking on them anywhere they appear in the shop HUD.