I think this "inventory slots upgrading" thing is a little confusing, so I decided to create this guide. I hope it helps to avoyd people from spending their mall points on an item that they will later find out that they can not use yet.

Basicaly, those "X Slots Inventories" work like upgrades. It is like:

Normal 24 Slots Inventory (the one every player have) = an inventory without upgrades (Inventory 1.0)
28 Slots Inventory = First upgrade for your inventory (Inventory 2.0)
32 Slots Inventory = Second upgrade for your inventory (Inventory 3.0)
36 Slots Inventory = Third upgrade for your inventory (Inventory 4.0)
40 Slots Inventory = Fourth upgrade for your inventory (Inventory 5.0)
44 Slots Inventory = Fifth upgrade for your inventory (Inventory 6.0)
48 Slots Inventory = Sixth (and last) upgrade for your inventory (Inventory 7.0)

That means that, in order to upgrade your inventory to a "new version", you need to have the imediatly previous version. So, for example, before you upgrade your inventory for a 48 slots inventory, you need to have a 44 slots inventory. In other words, you can't simply upgrade your normal 24 slots inventory to a 48 slots one.

Total item mall points you need to spend to have a X slots inventory:

24 Slots Inventory --> Nothing - its the standard one
28 Slots Inventory --> 71 points (items you need to buy: 28 Slots Inventory)
32 Slots Inventory --> 134 points (items you need to buy: 28 Slots + 32 Slots Inventories)
36 Slots Inventory --> 189 points (items you need to buy: 28 Slots + 32 Slots + 36 Slots Inventories)
40 Slots Inventory --> 244 points (items you need to buy: 28 Slots + 32 Slots + 36 + 40 Slots Inventories)
44 Slots Inventory --> 299  points (items you need to buy: 28 Slots + 32 Slots + 36 + 40 + 44 Slots Inventories)
48 Slots Inventory --> 354 points (items you need to buy: 28 Slots + 32 Slots + 36 + 40 + 44 + 48 Slots Inventories)

Item mall poins needed for each upgrade: