I noticed that the second time I wanted to use an achievement item. Clicking on it and then the ok on the warning box, marked the item as bought, but the other items were not "greyed out".  That meant I could keep buyng them. Initially I thought this was a bug as it would allow me to use more than one of them. But then I noticed that they didn't appear in the rightmost slot at all. Apparently to equip one, you need to click on that item again once it's been bought (and marked). This is not really intuitive imho.

An item should be equiped as soon (and the selection menu closed) as soon the player clicks the "OK". As it is, the second "selection" of the bought item does not make any sense as you can't either equip more than one, or switch between the ones you've bought once you've equiped one of them.

As a sidenote, I'd also like to have the location where you need to click to get to that buying menu a bit more intuitive. Nobody thinks of clicking on an empty item slot unless they already know they have to. Perhaps we can fill this slot with a "dummy item" icon to draw the attention of the player, who upon clicking at it, will be taken to the selection menu.