The example I posted was just what a hidden form element that will set the value that is being requested from you by the bank should look like. The value that it is being set to is the total of the cart times 100. This total is already held in a session variable, I'm just assigning that value to the hidden form element.
Is the problem with the checkout button a new one? Please post back and include some more details about what is happening and what you are expecting as a result.