Paypal 10501 Failure in Transaction Invalid Configuration Error

Recently, we want to setup our store such that it should accept credit card payments without leaving our domain. The process is quite different compared to accepting payment via paypal's express checkout where the user is redirected to paypal to make the payment.

Unfortunately, we keep on encountering an error, which is:

Array
(
    [TIMESTAMP] => 2010%2d03%2d29T11%3a20%3a33Z
    [CORRELATIONID] => c4221526ca2b3
    [ACK] => Failure
    [VERSION] => 51%2e0
    [BUILD] => 1236498
    [L_ERRORCODE0] => 10501
    [L_SHORTMESSAGE0] => Invalid%20Configuration
    [L_LONGMESSAGE0] => This%20transaction%20cannot%20be%20processed%20due%20to%20an%20invalid%20merchant%20configuration%2e
    [L_SEVERITYCODE0] => Error
    [AMT] => 0%2e01
    [CURRENCYCODE] => USD
)

At first I thought there was something wrong in the parameters that I'm sending via the paypal's DoDirectPayment method, so I keep on debugging. After a while I got tired and search the internet, in this url the error is explained: https://cms.paypal.com/us/cgi-bin/?cmd=_render-content&content_ID=developer/e_howto_api_nvp_errorcodes

10501
Invalid Configuration
This transaction cannot be processed due to an invalid merchant configuration.
Occurs when the billing agreement is disabled or inactive.

And there was not a single hint on how to fix this. Digging magento's sandbox website, I've found it, finally.

1.) Login to sandbox: http://developer.paypal.com
2.) Click the "Merchant Services", 4th tab
3.) In the left panel select "Website Payments Pro"

4.) Finally, select "Accept Billing Agreement"

Promise, you need this :-D.