I've already responded to you via email, but in case anyone else is reading along here, I'll copy that reply:
The fields we report in the Payment Notification section of our Sale Notification emails are just a straight copy of the IPN we received from PayPal, and our Integration submissions are intended to match the format of a PayPal-style IPN (with a few extra fields), so I think any fields reported in the Sale Notification email should also be included in our Integration submissions.
You can also perform a test checkout using a different PayPal account or a credit card not associated with your PayPal account (you can just refund the payment immediately after testing), to see what fields are actually sent to your Integration URL. You might want to test this with a simple, generic form-handler script as your Integration URL, so you can get an email showing all the fields submitted to the script URL.
All order information is already captured in our database and accessible via your E-junkie Seller Admin > View/Download Transaction Log. However, if you also have a need to capture the order data in your own database as well for some reason, both 'txn_id' and 'invoice' should be unique strings for every order.
We recommend using 'txn_id' whenever possible, as that corresponds to the ID of an actual completed payment item in your PayPal (or other payment processor) account history, so 'txn_id' serves both as a 'proof of purchase' and as an easy cross-reference between your and the buyer's PayPal account histories, the payment receipt email the buyer receives from the payment processor, your E-junkie Transaction Log, the buyer's thank-you/download page URL, and in this case your own database records.
If you want to use a reference that doesn't correspond directly to a payment or anything else, you could use 'invoice' in that case.