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Basically, before you buy an item, you click the slot you want it in to highlight the slot. Purchased item is placed there. I know you can click and drag out of the store (personally, a little counter intuitive I find) but I have my in game HUD so shrunken that it's awkward to click and drag with the in-game cursor. Honestly, the first time I shopped, this is what I tried, and was a bit surprised to realize this feature was not available.