First trick is not to use USA as the ticket collection country, or you will automatically redirected to Raileurope, and unselect the "Direct Trains" box. The second trick is to register your Visa card with Visa Checkout or Mastercard is its equivalent.
The third trick is to skip those two steps and buy the ticket from www.capitainetrain.com, which has the exact same prices without the credit card vetting issues. The last trick is to notify your credit card company in advance that you will be making a foreign transaction so they don't refuse the transaction for suspected fraud. Tickets for Sept 21 should go on sale on June 21.