Checking out with and without Payment Gateway

I created a checkout form with the Payment Gateway included, but the shopping cart is for FREE stuff. The reason is that I also added a Donation option so that the visitor can simultaneously make a donation if he so wishes. The problem I have right now is that whenever no donation is made, there's no $ for the Paypal to process so I get an error because it can't charge 0. How do I make it so that when the visitor does not check the donation box, the payment Behavior is not active, but when they check the box to make a donation, the payment Behavior becomes active? In order other words how do I deactivate the ecart local checkout depending on whether the donation checkbox is not checked. That way it will process the transaction, put the data in the dbs and return the email receipt, but without going through the Payment gateway so I don't get an error.

Also, I want in a form to copy the fields from the billing info to the shipping info automatically (so that I can hide the shipping info when there is none required). I found some java code that works, except on the SELECT fields, such as as State and Country it only changes when there's some invalid entries and it bounces back to the form. But I need it to automatically change to the State I select in the Billing form after I tab out of it (onblur)....It doesn't work. Any suggestions?

Thank you.