Nobody?

You should place it in the config folder. Otherwise create a serviceprovider that publishes your config file to the config directory.

Not 100% sure, but I don't think you can call config on a file outside the config folder itself. How about storing your configuration variables inside the .env file? Then you could just make a providers.php file in the config folder, and reference the variables there.
.env

...
PAYPAL_USERNAME=xxx
PAYPAL_PASSWORD=xxx
PAYPAL_SIGNATURE=xxxx
...

config/providers.php

return array(
  'paypal' => [
    'username' => env('PAYPAL_USERNAME', 'xxx'),
    'password' => env('PAYPAL_PASSWORD', 'xxx'),
    ...
  ]
);