What is the best way to possess the order information (Order ID, Product ID, Price etc) when the visitor is redirected to my website to download the product file, right after a successful order at Plimus?

Since I have not been able to get reliable ecommerce tracking with Analytics or AdWords by inserting the code into my buynow pages, I have decided to have the visitor download the product from my website after the order at Plimus.

The visitor can come to my website via the 'continue' link after the buynow at Plimus, or via the link in the e-mail.

Now when the visitor is at my website after the purchase, I want to know which product she purchased so I can deliver the correct file.

(I figure this can be done with a simple parameter in the email URL , but I can only pass a few parameters that way before it starts becoming ugly)

Apart from this, I also want the Product Number, Invoice ID, Invoice Total etc so I can do a pageTracker._trackTrans(); for Google Analytics.

As I see it, I have three options:
#1) Pass everything as GET parameters in the download link sent by e-mail
Trouble with this is, it gets too messy, and is insecure.
#2) Pass everything as GET parameters in the "Return to URL"
Trouble with this is, I don't think PRODUCT_NUMBER is available as a parameter at that point.
#3) Use IPN to save everything to a database, and retrieve the values from that database, based on the Invoice ID which I can pass as a GET parameter as in #1.
This seems like it will work with only a few security issues, but it seems like a really roundabout and complex way to do this?

What do you suggest?