For all those who are intereted in paying the whatsapp yearly subscription:

You have to complete this link:

h*ttp://www.whatsapp.com/payments/cksum_pay.php?phone=49123456789&cksum=254c93800636d400c83c39614a5052f7

Where phone is you phone number in internation format (whatsapp's 'login name')
And cksum is the md5 hash of your phone number + 'abc'.

Taking the phone number "49123456789" as the example, you have to calculate the md5 hash for the following value "49123456789abc" (without the quotation marks). The md5 for 49123456789abc is 254c93800636d400c83c39614a5052f7

All you have to do then is follow the instructions in the browser.