Buy em all

Features
BuyEmAll enhances the shift-click interface at vendors.

Type in the amount you want to buy
This is actually part of the default UI. Enough people don’t know about it due to the fact that there is no cursor and clicking in the box gives no feedback, that I decided to list it here.

Buy more than the stack size of the item at once
For instance, if you want to buy 80 pieces of Silk Thread, you currently have to shift-click the thread, enter 20, hit Okay, shift-click the thread, enter 20, hit Okay, shift-click the thread, enter 20, hit Okay, shift-click the thread, enter 20, and hit Okay.

With BuyEmAll, you shift-click the thread, enter 80, hit Okay, and confirm that you want to buy more than a stack if the confirm is turned on. While you can disable the confirmation, I wouldn’t recommend it unless you’re careful with your purchases.

To toggle the purchase confirmation box use the slash command: /buyemall confirm

Know how much you will be spending

Below the box where you enter the amount is a money display. This updates as you change the amount you are going to buy, showing you exactly how much your purchase will cost.

Quickly fill stacks or buy as much as you can
The BuyEmAll window has Stack and Max buttons which allow you to enter a full stack or as much as you can afford/fit. Hovering over the buttons shows a tooltip with the number of items you will be buying and how much it will cost. See Notes for information on the maximum you can buy.

If you already have some of an item, the first click of the Stack button will enter the amount you need to fill that stack. Subsequent clicks will add full stacks to your purchase. Also, right clicks will decrease the amount by one full stack.

Notes
Maximum Purchase
The amount you can enter is limited by:

• The amount of free space in your bags. Partial stacks are included in this calculation, as are specialty bags (e.g. free slots in enchanting bags will be only be counted towards your free space if you are buying an essence, dust, or shard).

• The amount you can afford.

• The amount available from the vendor (for limited stock items). A breakdown of these numbers is included in the Max button’s tooltip.
  If you can only buy one item for any of these reasons, shift-click behaves just like a left click, picking up one of the item onto your cursor. If you can’t buy any, then shift-click does nothing.

Cataclysm
Due to the API for purchasing items from vendors changing, preset stacks effectively are no different than normal items to addons. So, no need to worry about preset stacks anymore, whichever number you enter, is how many items you will get. Usually that is.