Repeat clicking on wpf button add its own value

Question

I have a project vending machine. C#/ WPF
But i can only click once on the button it needs to be clicked on till the value is the same as the cost
Example if the drink cost 2€
I can only click once on the button 1€
The 2€ button is not a problem cauze this value is the same and proceeds to the next step.
an image of the project http://img338.imageshack.us/img338/5613/softdrinkvend.jpg