Apparently that's the only coinmaster where you can't specify how many you want to buy, so items need to be purchased one at a time. It looks like that should be covered by the lack of a specified quantity field, but apparently that isn't working somewhere along the way.
Why was it trying to buy 11 of 10 items? Is that an improperly batched buy?
That seems to happen just from normal use of the coinmaster interface.