Payflow Link is a payment solution provided by PayPal (they acquired it with Verisign) but still maintained separately from Website Payments Standard. It is a hosted solution, meaning customers will be redirected from your checkout review page to the payment page to enter their credit card details. This module requires you to have a Payflow Link account which has significant startup fees but fairly low operating costs.
To test the module, use the following settings in your Payflow Link account:
I kept all the address fields unchecked and tweaked the other settings for the Livetest. Of course, replace example.com with your site's address. You can test this on the Livetest if you want by selecting "Credit card on a secure server".
When operating in test mode, the order total determines the response you will receive from PayPal. Complete checkout for any order under $1000 to test with an accepted payment.
When you're ready to process real transactions, simply change your transaction process mode to Live in your Payflow Link account settings.