Did one of the items you bought come with a free bonus item offer, like a game or accessory? If so, that would appear on your invoice as "DISCOUNT FOR AUTOADD #(number.)"
If you buy an item worth $200 and get a free gift worth $20, they will charged you $180 ($200 with $20 AUTOADD discount) plus $20 for the free gift. The reason why they do this is so that if you request a refund, they will only refund you $180. They do not want you to use the free gift and then ask for your money back.