I'm trying to send invoices to my customers through the Amazon MWS API. I know how to get the buyer's information for creating the invoice (PDF), but how can I send it to my customers through Amazon?
You can use the GetOrder operation on the Order API.
With this operation, you can obtain the buyer contact information (BuyerEmail, BuyerName, ShippingAddress).
For a standard order, this information is available only if the order status is Unshipped, PartiallyShipped or Shipped. You won't get the buyer email if it's an Amazon gift.