I purchsed Production Assistant 2 in mid-2012 from Sony Creative Software, but the purchase does not appear on My Account on either SCS or MAGIX's websites. Did you copy the email receipt containing the serial number by any chance as that would contain the purchase information?
However, if it was Production Assistant 3 purchased through MAGIX and it does not appear in your MAGIX My Account purchases, perhaps contact MAGIX directly via their Support avenue.
As Dexcon says. The standard Support form suggested answer: "Q. I need an invoice: A. When you make a purchase in the MAGIX Shop, you will automatically receive an invoice. If you purchased a download version, activation or service an invoice will be sent to you by email the following working day. (Please make sure to check your Spam folder if necessary). If you have purchased an order in which the items are sent by post a paper invoice will be enclosed with the item."