Hi Luke Mueller,
I have another alternative, with can give more information to the customer when they input the qty into the cell. I think the Menu>Data>Validation is a proper one to solve your problem.
With data validation....
1) customer can get the restriction / information, how to before input wrong qty.
2) when the qty input is not correct then they get the error message to inform and suggest them to next step.
3) the quantity will always be correct, with style "Stop", the wrong qty will be blocked.
Hope this help..