The issue:
If you are using PayPal Express Checkout and keep getting the error "security header invalid" during checkout when clicking on PayPal Checkout.
The cause:
You cannot use a live PayPal API signature in test mode, you need to use the PayPal sand box API for that. In order to use test mode, you must setup a PayPal Sandbox account, generate the signiture there and use the signature from your sandbox account in your PayPal Express Checkout settings in CS-Cart.
