If I understand you correctly, you are really looking to sell standalone digital files that customers either print out somewhere on their own or just keep as a finished download.
I honestly think Zazzle might be the wrong platform for that. This marketplace is built around finished physical goods. Zazzle routes the print files directly to manufacturing partners, and shoppers come here looking for tangible paper products.
You do see some hybrid options where an invitation is offered both in print and as an instant digital download. Zazzle also seems to be expanding into fully digital event stationery with RSVP tracking and related online features. But selling pure downloadable files as your primary product is simply not what this site is set up for today.
With planners, for example, you can only customize the outer cover, not the interior layout. There are notepads with 40 identical tear off pages and posters, but in the end, those are still physical products meant to be printed and shipped, not just handed over as a file download.