I was charged twice for an online purchase?

So the other day I bought two different items from two different sellers on Amazon; I used my American Express card. Then a couple days later when I get notified of the shipments, I get charged again. So basically I have payed twice for these two purchases which is like $20 more. I asked an AMEX agent about the situation and he said that I was just overcharged and the money would go back to my account after the merchants "released funds." But it has been a few days now and I am getting a little worried. Maybe I'm just confused, could someone explain this to me?