I have the answer! I too had a series of weird charges from AMZN.COM/BILLWA (freaked me out!) and so I talked to an Amazon customer service rep this morning. Turns out these weren't fraudulent charges. Here's what happens:
Amazon does not charge your card immediately when you put in an order. Your card is billed only when the package goes out the door on the shipping truck (which generally happens days after you make your purchase).
When your shipment comes in 2 or more boxes from different centers around the country, the charges will split into the corresponding amounts.
So let's say you ordered X, Y and Z for a total of $100 on Monday.
Items X and Y, $60 worth of stuff, ships Wednesday.
Item Z, worth $40, ships Thursday.
That means your card is charged $60 on Wednesday, and $40 on Thursday. It is NOT charged $100 on Monday.
It's a weird system, but that's apparently how Amazon does it. Go back and cross-check your order history with your card statement. Things should add up correctly. If not, then it might be a different problem you've encountered.
If you're still concerned, call the Amazon customer service line at 866-216-1072.