api - Express Checkout error message: "Security header is not valid"

Question

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?

Answer 1

https://api-3t.sandbox.paypal.com/nvp requires different user-accounts than what https://api-3t.paypal.com/nvp requires.

The sandbox accounts can be activated at developer.paypal.com