First sign in (or sign up if don't have an account yet).
Then click the Settings link in the top-right corner of the screen. You can then choose for how long to buy a Premium Service and pay by PayPal.
The prices for a Premium account are:
You can have the Premium Service for 1 month for free! Just activate the trial period. You can do it after you log in - click Settings, select the Payments tab and click the "Activate Trial" button.