Okay. This is becoming more clear.
These are the options that I see for confirmations (that don't include email):
1. The default Thank You page that has a download "link" when "Download" is checked in the administration area.
2. The default redirect page links to a predefined/customized Thank You page that can be hosted anywhere.
3. Custom HTML from the General Thank You page is appended to the code in the default Tha nk You page.
4. Custom HTML for the Product Thank You page is appended to the code in the default Thank You page, and follows HTML from the General Thank You HTML if it exists.
5. Custom HTML for the General Thank You and Product Thank You page will both ALSO appear in the iframe of a customized self-hosted Thank You page in the same appending order as it would appear in the default thank you page if we want to dynamic variables from e-junkie to appear on the page.
Understanding all of this, I'd still like to have the download button on my custom thank you page. It looks like the item_info DIV for the download link on the download page uses a form that passes two variables (txn_id and d_id) to a form called df.php.
"txn_id" is one of the parameters that can be passed to the custom thank you page within the iframe, but I can't find any reference to d_id.
So, at long length, my question is "What is d_id?". I suspect it to be an abbreviation for dealer ID, which would mean I could add it as a constant when I call the form to generate the download.
If it's not a constant, then I'll just give up on the custom download button.