I've been having real trouble capturing payment data on my own website via the Payment Variable Information URL. The reason appears to be that the e-Junkie user interface won't let me type the correct URL into the field for anything other than the first product I'm selling.
For the first product my "Payment Variable URL" looks like this:
Notice the "?&" in between the o_plug.php and the "postto=myurl"
Now, for any subsequent product I sell the UI simply won't let me type a "?&" in the field at all. If I copy/paste the URL from my first product it strips out those characters.
If you're willing, can you try to edit the Payment Data URL in the Seller Admin on your account and tell me if you can type those characters? Remember, for me it works for my first item for sale but not on any subsequent items.