the error coming back from payapal is:
Security header is not valid
This error means there is an issue wit hthe API Credentials you have entered.
the three most common causes of this error are:
1) Using Live API Credentials, but sending the request to the Sandbox
2) Using Sandbox credentials but sending the request to the live server
3) Not excepting the paypal billing agreement
make sure the Use Sandbox for testing option is set correctly in the Authorize transaction with Express Checkout server behavior on the checkout page.
if you are certain the api credentials are correct and the sandbox option matches, you may just have bad api credentials, try requesting new ones.
