It's only 1 extra point per dollar, so in most cases it is double. However, you'd be better off getting a card with travel rewards and not paying the additional fees per booking to Amex Travel. If you want to stay with Amex, get the Premier Rewards Gold card for 3x points on airfare booked with airlines. However, there are other options out there like the Chase Sapphire Preferred or Citi Prestige which earn 2x-3x points on various travel purchases.

If you don't want to get another card, then the Amex portal is most likely not worth it. They charge a booking fee that would usually outweigh the additional points.