Yes, you can simply paste that AdWords conversion tracking code into your E-junkie Seller Admin > Edit Preferences > Common Thank-you Page HTML field if you want it inserted for every sale regardless of the items ordered. However, if you only want it inserted when a specific product is ordered, you would paste that into the Product Thank-you Page HTML field of that product's settings. Whatever HTML you provide in either of those fields would be inserted directly into the raw HTML source of the thank-you/download pages we generate for you, so buyers would not see that code.