That option has been disabled.
I don't care for the way it looks and took it out of the site cart software.
For now, you'll have to do the "ten or less" route.
Add 10 to your cart, then keep subtracting one from the total until you have the amount left in stock.
I say 10 - because there is only one "Army Builder" that is over that amount.
Sucks, I know. But I'm not thinking of the "army building" folks when I set up the site, I'm thinking that you're missing a figure, weapon or item, and that you'll enter in how many you need - if there's enough, then you can checkout, if not, you adjust your order.
I am working on changing part of the cart to say how many is left if enter an amount higher than what is in stock - but I don't think I'll have that done anytime soon... and might even be sold of of whatever it is you two are looking at in that time.
