I'm implementing Express Checkout in PayPal.
I have no problem with the first two steps, SetExpressCheckout and GetExpressCheckout. But when I use DoExpressCheckout, I encounter the error "Security header is not valid".
The API credentials are the same!
I've fixed it by changing the $environment to live in DoExpressCheckout. (The difference $environment makes is that it'll use https://api.sandbox.paypal.com/nvp/ instead of https://api-3t.$environment.paypal.com/nvp)
But why?
Is there something wrong with https://api-3t.$environment.paypal.com/nvp?