Error during purchase: 'Invalid URL'; 'We can't find this page'

When I try to purchase in-game currency, I receive this error message:

This occurs when customers have disabled their cookies. To avoid this error message from popping up every time you make a purchase, you will need to change your browser settings to allow third-party cookies, or at least from TrialPay. 

How do I enable cookies?

Chrome (box should be unchecked):


 

Firefox (box should be checked):



 

Safari: