Bukkit

Multiple Items can be at the same slot of an inventory. All Items are loaded in a list, but just the first one is displayed. You can either use the Addon BS-SingleUse or the "HideItemsPlayersDoNotHavePermissionsFor" Option in the BossShop Config if you want to work with overlapping items.

BS-SingleUse hides Items after they were purchased by the player x-times (Configurable), while the "Item Hiding" Option hides Items if the player does not have permissions for them.

When an Item is hidden, the next Item of the list is displayed (if there is one).

Example:

Items:

• Upgrade 1
  • InventoryLocation 5
  • SingleUse: 1
  • PriceType: Money
  • [...]
• Upgrade 1
  • InventoryLocation 5
  • SingleUse: 1
  • PriceType: Money
  • [...]
• Upgrade 2
  • InventoryLocation 5
  • SingleUse: 1
  • PriceType: Money
  • [...]
• Upgrade 3
  • InventoryLocation 5
  • SingleUse: 1
  • PriceType: Money
  • [...]

In this example, first the Item "Upgrade 1" is shown. After it is purchased for money, it is hidden (By the SingleUse Addon). Now "Upgrade 2" is displayed. After "Upgrade 2" is bought, it is hidden and now "Upgrade 3" is shown. I think you got how it works now.